Methods for liquid extraction of rare earth metals using ionic liquids

ABSTRACT

A method for extracting a rare earth element from a rare earth-containing substance, the method comprising mixing the rare earth-containing substance with a protic ionic liquid, such as: 
     
       
         
         
             
             
         
       
     
     wherein R 1  is selected from hydrogen atom and hydrocarbon groups containing 1 to 6 carbon atoms; R 2  and R 3  are independently selected from hydrocarbon groups containing 1 to 12 carbon atoms; and X −  is an anionic species; to produce a composition of the formula (RE)(amide) y X z  at least partially dissolved in the protic ionic liquid, wherein RE is at least one rare earth element having an atomic number selected from 39, 57-71, and 90-103; y is 2-6; z is a number that charge balances the total positive charge of RE; and the amide is the conjugate base of the cationic portion of the protic ionic liquid of Formula (1) and has the following formula:

CROSS REFERENCE TO RELATED APPLICATION

The present application claims benefit of U.S. Provisional ApplicationNo. 62/151,567, filed on Apr. 23, 2015, all of the contents of which areincorporated herein by reference.

GOVERNMENT SUPPORT

This invention was made with government support under Prime Contract No.DE-AC05-00OR22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to methods for extracting rareearth elements from a rare earth-containing substance, and moreparticularly, to methods in which such rare earth elements are extractedinto one or more ionic liquids.

BACKGROUND OF THE INVENTION

Rare-earth elements play an important role in modern technologiesincluding, but not limited to, high-strength magnets andenergy-efficient fluorescent lighting. The rare earths (REs) typicallyencompass 15 members of the 4f row (lanthanides, Z=57-71) and asecond-row transition metal, yttrium (Z=39). The rare earths may also beconsidered to encompass the actinide elements (Z=89-103). Many of therare earths occur in nature except for man-made promethium (Z=59) andseveral of the actinides. The lanthanides and yttrium are allthermodynamically stable trivalent ions in solution and the solid state.In addition, a phenomenon known as the lanthanide contraction isobserved. This phenomenon displays an incremental decrease in the radiusof the RE³⁺ ion from La³⁺ to Lu³⁺. As a result of the stable 3+oxidation state and the small changes in ionic radii between adjacentlanthanides and actinides, separating rare earths is challenging. Inaddition to lanthanides and actinides having such similar solutionchemistries, the rare earths also require their ores to be processed,which has a number of industrial challenges.

The demand on industry to provide an accessible supply of thesematerials is ever-increasing as rare earths continue to play a moreimportant role in applications related to modern technology. As of 2011,the majority of world rare-earth element production is in China (95%),with Australia (2%) and India (2.5%) contributing substantially less,and Brazil and Malaysia contributing the remainder (0.47%) (e.g.,Gschneidner Jr., Mater. Matters (Aldrich) 2011, 6, 32-37). In China andthe U.S., bastnaesite is the mineral of most interest for rare-earthrecovery. Bastnaesite, a rare-earth carbonate fluoride (RECO₃F) mineral,is approximately 7-10% rare-earth oxide (REO) consisting mostly of thelighter elements (ca. 98% La—Nd) (e.g., J. B. Hedrick, “Rare Earths,” inMetals and Minerals, U.S. Geological Survey Minerals Yearbook 2001, vol.I, 61.1-61.17).

Historically, bastnaesite has undergone a series of physicochemicalprocesses to produce a commercial product. Considering the flow sheetfor bastnaesite employed by Molycorp (F. F. Aplan, The processing ofrare-earth minerals. The Minerals, Metals and Materials Society,Warrendale, Pa., USA, 1988), the following generalized process may bewritten:

Bastnaesite ore (7% REO)→crushing/grinding→conditioning→series offlotation steps→leaching→calciner→bastnaesite→concentrate (90%REO)→separation plant

After this beneficiation process, chemical treatment of either the crudeore or the bastnaesite concentrate may take place, e.g., P. R. Kruesi,G. Duker, Min. Met. Mater. S 1965, 17, 847. The process involvesleaching bastnaesite with hydrochloric acid, treating the resultingrare-earth fluorides (REF₃) with sodium hydroxide, and finally,solubilizing the rare-earth hydrolysis product with hydrochloric acid.In general, this process employs the following steps:

3RECO₃F+9HCl→REF₃+2RECl₃+3HCl+3H₂O+3CO₂

REF₃+3NaOH→RE(OH)₃+3NaF

RE(OH)₃+3HCl→RECl₃+3H₂O

However, this chemical treatment process requires heating of the ore andindustrial solutions to roughly 95° C. for 4 hours at different stagesand the consumption of 2.5 kg HCl/kg of RE₂O₃ and 0.73 kg NaOH/kg REOfeed to achieve the final product. Considering the significantexpenditures in energy, time, and cost for current rare earth extractionmethods, there would be a significant benefit in a simplified and lesscostly process for achieving the same end.

SUMMARY OF THE INVENTION

The invention is directed to methods for extracting one or more rareearth elements from a rare earth-containing substance by mixing the rareearth-containing substance with one or more protic ionic liquids thatform a complex with one or more of the rare earth elements, wherein thecomplex is at least partially soluble in the protic ionic liquid and/ora solvent (e.g., a non-aqueous solvent) in which the protic ionic liquidis dissolved. The invention is also directed to methods for at leastpartially separating one rare earth element from another rare earthelement, particularly among lanthanide elements, from a rareearth-containing substance containing a mixture of such elements. Theinvention accomplishes this separation by virtue of an ability of theprotic ionic liquid to better dissolve one or more rare earth elementscompared to one or more other rare earth elements.

In one set of embodiments, the protic ionic liquid includes a protonated(i.e., positively-charged) carboxamide-containing species. The proticionic liquid can be conveniently expressed by the following formula:

wherein R¹ is selected from hydrogen atom and hydrocarbon groupscontaining at least 1 and up to 12 carbon atoms; R² and R³ areindependently selected from hydrocarbon groups containing at least 1 andup to 12 carbon atoms; and X⁻ is an anionic species.

When using the protic ionic liquid according to Formula (1), the mixingprocess results in a solution containing a rare earth composition of theformula (RE)(amide)_(y)X_(z) at least partially dissolved in the proticionic liquid and/or a solvent in which the protic ionic liquid isdissolved, wherein RE is at least one rare earth element having anatomic number selected from 39, 57-71, and 90-103 and having a positiveoxidation state; y is generally 2-6; z is a number that serves to chargebalance the total positive charge of the at least one rare earth metals(RE); X is equivalent to X⁻ in the ionic liquid of Formula (1); and theamide is the conjugate base of the cationic portion of the protic ionicliquid of Formula (1) and has the following formula:

In another set of embodiments, the protic ionic liquid is a protonated(i.e., positively-charged) phosphine oxide-containing species. Theprotic ionic liquid can be conveniently expressed by the followingformula:

wherein R⁴, R⁵, and R⁶ are independently selected from hydrocarbongroups (R) containing at least 1 and up to 12 carbon atoms and alkoxidegroups —OR; and X⁻ is an anionic species.

When using the protic ionic liquid according to Formula (4), the mixingprocess results in a solution containing a rare earth composition of theformula (RE)(phos)_(y)X_(z) at least partially dissolved in the proticionic liquid or a solvent in which the protic ionic liquid is dissolved,wherein RE is at least one rare earth element having an atomic numberselected from 39, 57-71, and 90-103 and having a positive oxidationstate; y is generally 2-6; z is a number that serves to charge balancethe total positive charge of the at least one rare earth metals (RE); Xis equivalent to X⁻ in the ionic liquid of Formula (4); and the phosgroup is the conjugate base of the cationic portion of the protic ionicliquid of Formula (4) and has the following formula:

The above-described methods represent a significant advance in theextraction of rare earth metals at least in view of their simplificationof the process and the associated lower energy demand and diminishedcost. The ability of the described methodology to also selectivelyextract one or more rare earth elements from one or more other rareearth elements represents a further significant advance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Structures of the cationic and anionic components of the ionicliquids used in the instant application for extraction of rare earthmetals.

FIG. 2. Graph showing dissolution of rare earth (RE) minerals by 2mol/kg DMAH⁺NTf₂ ⁻ in BMIM⁺NTf₂ ⁻ at 120° C. and 1000 rpm. Concentrationof RE³⁺, i.e., [RE³], shown at 1 hour (□) and 24 hour (▪) contact times.The symbol ▴ represents the % RE dissolved at 24 hours. Error bars aretwice the standard deviation at 95% confidence.

FIG. 3. Powder X-ray diffraction (PXRD) patterns labeled as 3a-3e asfollows: synthesized (3a) NdCO₃F, (3b) CeCO₃F, (3c) PrCO₃F, (3d) LaCO₃F,and (3e) natural bastnaesite (La).

FIGS. 4A, 4B. FIG. 4A shows FTIR-ATR spectra of synthetic bastnaesiteand RECO₃F solids, while FIG. 4B compares the FTIR-ATR spectra of RECO₃Fwith RE₂(CO₃)₃ solids. Designations are as follows: (4a.1)Bastnaesite-Mtn Pass Blend, (4a.2) Bastnaesite-10% Heavies Blend, (4a.3)LaCO₃F, (4a.4) CeCO₃F, (4a.5) PrCO₃F, (4a.6) NdCO₃F, (4a.7) TbCO₃F,(4a.8) DyCO₃F, (4a.9) HoCO₃F, (4a.10) YCO₃F, (4b.1) TbCO₃F, (4b.2)YCO₃F, (4b.3) Tb₂(CO₃)₃-xH₂O, and (4b.4) Y₂(CO₃)₃-xH₂O.

FIG. 5. PXRD patterns of HoCO₃F post-dissolution, designated as (5a)HoF₃ with literature comparison (5b) and NdCO₃F post-dissolution as (5c)NdF₃ and the literature comparison (5d) after 5 hours.

FIG. 6. Dissolution of NdCO₃F (□), DyCO₃F (⋄), and HoCO₃F (Δ) forvarying total amounts of RECO₃F in a system in contact with 2 mol/kgDMAH⁺NTf₂ ⁻ in BMIM⁺NTf₂ ⁻ at 120° C. and 1000 rpm for 5 hours. A dashedline with a slope of ⅔ is provided for comparison.

FIG. 7. Proposed reaction scheme for the dissolution process in the ILsystem. Lower left circles indicate initial reactive solution species,rectangles indicate solid phases, upper left circles indicate speciesthat exist in the system as gases, and the two rightmost circlesindicate RE solution species.

FIGS. 8A, 8B. FIG. 8A shows initial (bottom) and final (top) absorbancespectra and FIG. 8B shows speciation of Nd³⁺ in BMIM⁺NTf₂ ⁻ at 120° C.The following species are present: 0: Nd³⁺; 1: Nd(DMA)³⁺; 2: Nd(DMA)₂³⁺; 3: Nd(DMA)₃ ³⁺; 4: Nd(DMA)₄ ³⁺; 5: Nd(DMA)₅ ³⁺. ExperimentalNd³⁺/DMA ratios range from 1:0 to 1:7.25. Note: the Nd spectra areshifted in order to display the features of each.

FIG. 9. Graph showing enrichment factor [quotient (□) of the heavy tolight ratio in 2 mol/kg DMAH⁺NTf₂ ⁻ in BMIM⁺NTf₂ ⁻ divided by the ratioof heavy to light REs in natural bastnaesite] as a function of time.Conditions: 45° C. and 700 rpm.

FIG. 10. Graph plotting ratio of heavy/light rare earths in 2 mol/kgDMAH⁺NTf₂ ⁻ in BMIM⁺NTf₂ ⁻ diluent as natural bastnaesite is dissolvedas a function of time. Bastnaesite (3 g) was dissolved in 62.3 g of IL.Conditions: 45° C. and 700 rpm.

DETAILED DESCRIPTION OF THE INVENTION

The extraction method can be applied on any rare earth-containingsubstance, particularly a solid substance containing one or more rareearth elements, typically in salt form, such as a carbonate, oxide,and/or halide of the at least one rare earth element. In particularembodiments, the rare earth element is selected from a lanthanide (i.e.,an element having an atomic number of 57-71) and/or an actinide (i.e.,an element having an atomic number of 89-103) or a subset of elementstherein. Some examples of lanthanide elements include yttrium (Y),lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium(Lu). Some examples of actinide elements include actinium (Ac), thorium(Th), protactinium (Pa), and uranium (U). In any of these embodiments,the rare earth elements may or may not also include yttrium (atomicnumber of 39). In some embodiments, the rare earth-containing substanceis a rare earth-containing mineral, such as bastnaesite, which, at leastin one of its forms, has the approximate composition (Ce, La)CO₃F. Aswell known in the art, bastnaesite may include other rare earthelements, typically the lanthanides with atomic numbers selected from 39and 57-71, and may also have a percentage of hydroxy (OH) groupsreplacing F atoms. In other embodiments, the rare earth-containingsubstance is waste or effluent emanating from, for example, a recyclingprocess, mining process, or nuclear energy process. The rareearth-containing substance may or may not also include one or morealkaline earth species (e.g., Mg²⁺, Ca²⁺, Sr²⁺, or Ba²⁺) and/or one ormore alkali species (e.g., Li⁺, Na⁺, K⁺, Rb⁺, or Cs⁺). In someembodiments, any one or more of the foregoing rare earth or otherelements are not present in the rare earth-containing substance.

In the method for extracting one or more rare earth elements from a rareearth-containing substance, the rare earth-containing substance is mixedwith one or more protic ionic liquids according to Formula (1) orFormula (4) that form a complex with one or more of the rare earthelements, wherein the complex is at least partially soluble in theprotic ionic liquid and/or a solvent (e.g., a non-aqueous solvent) inwhich the protic ionic liquid is dissolved. The mixing process should besufficiently intimate and rigorous so as to maximize the extractionpotential of the protic ionic liquid being used under the conditions(e.g., temperature and pressure) used. The intimate mixing can beachieved by any of the methods known in the art, such as by manual ormechanical stirring, vibrating, shaking, or tumbling. In someembodiments, high-speed stirring by use of a high-speed mixer is used.The high-speed mixer may provide a stirring speed of at least, forexample, 100, 200, 500, 1000, 1500, or 2000 revolutions per minute(rpm). The mixing process may also include grinding of the rareearth-containing substance either before or during contact with theprotic ionic liquid.

The mixing process may also employ any suitable temperature below thetemperature at which the protic ionic liquid decomposes. In differentembodiments, a temperature of about, at least, above, up to, or lessthan, for example, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 150, 180,or 200° C. may be employed, wherein the term “about” generally indicateswithin ±0.5%, 1%, 2%, 5%, or up to ±10% of the indicated value.Typically, the mixing step is conducted at standard pressure(approximately 1 atm); however, in some embodiments, an elevatedpressure may be used, such as a pressure of about or at least, forexample, 1.5, 2, 2.5, 3, 4, 5, or 10 atm, or a reduced pressure may beused, such as up to or below, 1, 0.8, 0.5, or 0.1 atm, or even up to orbelow 1000, 500, 200, 100, 50, 20, or 10 mTorr. A lower pressure, inparticular, could help evacuate the gaseous phases, such as water andcarbon dioxide, and leave the ionic liquid behind as well as the REproducts.

The protic ionic liquid functions to dissolve at least a portion of oneor more rare earth elements from the rare earth-containing substanceinto the protic ionic liquid or into a solvent in which the protic ionicliquid is dissolved. The extent of extraction can be quantified as theamount of rare earth metal extracted into solution vs. the total amountof rare earth metal originally present in the rare earth-containingsubstance. The foregoing proportion may also be termed the “extractionefficiency”. The extraction efficiency of the process is typically atleast 10%, and may be, for example, at least 15, 20, 25, 30, 35, 40, 45,50, 60, 70, 80, 90, 95, or 98%. In some embodiments, the protic ionicliquid extracts one or more rare earth elements to a greater degree(i.e., at a higher extraction efficiency) than one or more other rareearth elements, thereby enriching the protic ionic liquid or the solventin which it may be dissolved to a greater degree with one or more of therare earth elements as compared to one or more of the other rare earthelements. For example, in some embodiments, the rare earth-containingsubstance includes at least one of a first rare earth element selectedfrom lanthanum, cerium, praseodymium, and neodymium and at least one ofa second rare earth element selected from europium, terbium, dysprosium,holmium, and yttrium, and the protic ionic liquid dissolves at least oneof the second rare earth element to a greater degree than at least oneof the first rare earth element, thereby enriching the protic ionicliquid or a solvent in which it may be dissolved to a greater degreewith at least one of the second rare earth element than at least one ofthe first rare earth element.

As well known in the art, the term “ionic liquid” refers to an ioniccompound that is a liquid without being dissolved in a solvent. Theionic liquids considered herein are typically liquid at room temperature(e.g., 15, 18, 20, 22, 25, or 30° C.) or lower. However, in someembodiments, the ionic liquid may become a liquid at a highertemperature than 30° C. if the process is conducted at an elevatedtemperature that melts the ionic liquid. Thus, in some embodiments, theionic liquid may have a melting point of up to or less than 100, 90, 80,70, 60, 50, 40, or 30° C. In other embodiments, the ionic liquid is aliquid at or below 10, 5, 0, −10, −20, −30, or −40° C. The term “protic”(as in the term “protic ionic liquid”) refers to the presence of anacidic proton in the ionic liquid, as exemplified in the protic ionicliquids in Formulas (1) and (4)

In one set of embodiments, the protic ionic liquid contains a protonatedamide (i.e., amidium) species. These types of protic ionic liquids canbe expressed by the following formula:

In the above Formula (1), R¹ is selected from hydrogen atom (H) andhydrocarbon groups (R) containing at least 1 and up to 12 carbon atoms;R² and R³ are independently selected from hydrocarbon groups containingat least 1 and up to 12 carbon atoms; and X⁻ is an anionic species. Whenany of R¹, R², and R³ are hydrocarbon groups, the hydrocarbon groups mayindependently contain 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12 carbonatoms, or independently, a number of carbon atoms within a range boundedby any two of the foregoing number of carbon atoms (e.g., a number ofcarbon atoms of 1-8, 1-6, 1-4, or 1-3).

Moreover, it is apparent that the amidium species shown in Formula (1)is in flux with its tautomer, as shown in the following equation:

In some embodiments of Formula (1), R¹, R², and R³ are all hydrogenatoms. In other embodiments, at least R¹ is a hydrocarbon group, or atleast one or two of R¹, R², and R³ or at least one or both of R² and R³are hydrocarbon groups independently containing 1, 2, 3, 4, 5, 6, 7, 8,9, 10, 11, or 12 carbon atoms. In a third set of embodiments, all of R¹,R², and R³ are hydrocarbon groups independently containing 1, 2, 3, 4,5, 6, 7, 8, 9, 10, 11, or 12 carbon atoms.

In a first set of embodiments, one or more of the hydrocarbon groups (R)are selected from saturated groups composed solely of carbon andhydrogen and containing at least one carbon-hydrogen bond. Thehydrocarbon groups (R) can be selected from, for example,straight-chained alkyl groups, branched alkyl groups, and cycloalkylgroups. Some examples of straight-chained alkyl groups include methyl,ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl,n-decyl, n-undecyl, and n-dodecyl groups. Some examples of branchedalkyl groups include isopropyl (2-propyl), isobutyl (2-methylprop-1-yl),sec-butyl (2-butyl), t-butyl, 2-pentyl, 3-pentyl, 2-methylbut-1-yl,isopentyl (3-methylbut-1-yl), 1,2-dimethylprop-1-yl,1,1-dimethylprop-1-yl, neopentyl (2,2-dimethylprop-1-yl), 2-hexyl,3-hexyl, 2-methylpent-1-yl, 3-methylpent-1-yl, and isohexyl(4-methylpent-1-yl) groups, wherein the “1-yl” suffix represents thepoint of attachment of the group. Some examples of cycloalkyl groupsinclude cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl,and cyclooctyl groups. The cycloalkyl group can also be a polycyclic(e.g., bicyclic) group by either possessing a bond between two ringgroups (e.g., dicyclohexyl) or a shared (i.e., fused) side (e.g.,decalin and norbornane).

In a second set of embodiments, one or more of the hydrocarbon groups(R) are selected from unsaturated groups composed solely of carbon, orcarbon and hydrogen. The unsaturated hydrocarbon groups (R) can beselected from, for example, straight-chained alkenyl (olefinic) oralkynyl groups, branched alkenyl or alkynyl groups, aliphaticcarbocyclic groups, and aromatic carbocyclic groups. Some examples ofstraight-chained alkenyl groups include vinyl, propen-1-yl (allyl),3-buten-1-yl(CH₂═CH—CH₂—CH₂—), 2-buten-1-yl (CH₂—CH═CH—CH₂—),butadienyl, and 4-penten-1-yl groups. Some examples of branched alkenylgroups include propen-2-yl, 3-buten-2-yl (CH₂═CH—CH.—CH₃), 3-buten-3-yl(CH₂═C.—CH₂—CH₃), 4-penten-2-yl, 4-penten-3-yl, 3-penten-2-yl,3-penten-3-yl, and 2,4-pentadien-3-yl groups, wherein the dot in theforegoing exemplary formulas represents a radical or the point ofattachment of the group. Some examples of aliphatic carbocyclic groupsinclude cyclopropenyl, cyclobutenyl, cyclopentenyl, cyclopentadienyl,cyclohexenyl, and cyclohexadienyl groups. Some examples of aromaticcarbocyclic groups include phenyl and benzyl groups. The unsaturatedcyclic hydrocarbon group can also be a polycyclic group (such as abicyclic or tricyclic polyaromatic group) by either possessing a bondbetween two of the ring groups (e.g., biphenyl) or a shared (i.e.,fused) side, as in naphthalene, anthracene, phenanthrene, phenalene, orindene.

In some embodiments, the hydrocarbon group (R) group may include one ormore heteroatoms (i.e., non-carbon and non-hydrogen atoms), such as oneor more heteroatoms selected from oxygen, nitrogen, sulfur, silicon,phosphorus, boron, and halide atoms, as well as groups containing one ormore of these heteroatoms (i.e., heteroatom-containing groups). Someexamples of oxygen-containing groups include hydroxy (OH), alkoxy (OR),carbonyl-containing (e.g., carboxylic acid, ketone, aldehyde, carboxylicester, amide, and urea functionalities), nitro (NO₂),carbon-oxygen-carbon (ether), sulfonyl, and sulfinyl (i.e., sulfoxide)groups. Some particular examples of alkoxy groups —OR include methoxy,ethoxy, n-propoxy, isopropoxy, n-butoxy, isobutoxy, t-butoxy, phenoxy,benzyloxy, 2-hydroxyethoxy, 2-methoxyethoxy, 2-ethoxyethoxy, vinyloxy,and allyloxy groups. In the case of an ether group, the ether group canalso be a polyalkyleneoxide (polyalkyleneglycol) group, such as apolyethyleneoxide group. Some examples of nitrogen-containing groupsinclude primary amine, secondary amine, tertiary amine (i.e., —NR₂ or—NR₃ ⁺, wherein R is independently selected from H and hydrocarbongroups set forth above), nitrile, amide (i.e., —C(O)NR₂ or —NRC(O)R,wherein R is independently selected from hydrogen atom and hydrocarbongroups set forth above), imine (e.g., —CR═NR, wherein R is independentlyH or a hydrocarbon group), oxime (—CR═N—OH), amidoxime (—C(NH₂)═N—OH),nitro, urea (—NR—C(O)—NR₂, wherein R is independently H or a hydrocarbongroup), and carbamate groups (—NR—C(O)—OR, wherein R is independently Hor a hydrocarbon group). Some examples of phosphorus-containing groupsinclude —PR₂, —PR₃ ⁺, —P(═O)R₂, —P(OR)₂, —O—P(OR)₂, —R—P(OR)₂,—P(═O)(OR)₂, —O—P(═O)(OR)₂, —O—P(═O)(OR)(R), —O—P(═O)R₂, —R—P(═O)(OR)₂,—R—P(═O)(OR)(R), and —R—P(═O)R₂ groups, wherein R is independentlyselected from hydrogen atom and hydrocarbon groups set forth above. Someexamples of sulfur-containing groups include mercapto (i.e., —SH),thioether (i.e., sulfide, e.g., —SR), disulfide (—R—S—S—R), sulfoxide(—S(O)R), sulfone (—SO₂R), sulfonate (—S(═O)₂OR, wherein R is H, ahydrocarbon group, or a cationic group), and sulfate groups (—OS(═O)₂OR,wherein R is H, a hydrocarbon group, or a cationic group). Some examplesof halide atoms include fluorine, chlorine, bromine, and iodine. One ormore of the heteroatoms described above (e.g., oxygen, nitrogen, and/orsulfur atoms) can be inserted between carbon atoms (e.g., as —O—, —NR—,or —S—) in any of the hydrocarbon groups described above to form aheteroatom-substituted hydrocarbon group. Alternatively, or in addition,one or more of the heteroatom-containing groups can replace one or morehydrogen atoms on the hydrocarbon group. In some embodiments, any one ormore of the above heteroatoms or heteroatom groups may be excluded fromthe hydrocarbon (R).

The anion (X⁻) of the protic ionic liquid is any anion which, whenassociated with the protonated cationic component, permits the resultingionic compound to behave as an ionic liquid. As known in the art, thecomposition and structure of the anion strongly affects the properties(e.g., melting point, volatility, stability, viscosity, hydrophobicity,and so on) of the ionic liquid. In some embodiments, the anion isstructurally symmetrical, while in other embodiments, the anion isstructurally asymmetrical. Although the anion (X⁻) in the ionic liquidis typically monovalent (i.e., charge of −1), as depicted in Formulas(1) and (4), the anion may have a higher valency (e.g., charge of −2 or−3), in which case the stoichiometric ratio between the protonatedcation species and anion would be other than 1:1 in order for chargeneutrality to be preserved in the ionic liquid molecule. The anion (X⁻)is intended to encompass anions having any valency, unless otherwisestated.

In one embodiment, the anion (X⁻) of the ionic liquid isnon-carbon-containing (i.e., inorganic). The inorganic anion may, in oneembodiment, lack fluorine atoms. Some examples of such anions includechloride, bromide, iodide, hexachlorophosphate (PCl₆ ⁻), perchlorate,chlorate, chlorite, cyanate, isocyanate, thiocyanate, isothiocyanate,perbromate, bromate, bromite, periodate, iodate, dicyanamide (i.e.,N(CN)₂ ⁻), tricyanamide (i.e., N(CN)₃ ⁻), aluminum chlorides (e.g.,Al₂Cl₇ ⁻ and AlCl₄ ⁻), aluminum bromides (e.g., AlBr₄ ⁻), nitrate,nitrite, sulfate, sulfite, hydrogensulfate, hydrogensulfite, phosphate,hydrogenphosphate (HPO₄ ²⁻), dihydrogenphosphate (H₂PO₄ ⁻), phosphite,arsenate, antimonate, selenate, tellurate, tungstate, molybdate,chromate, silicate, the borates (e.g., borate, diborate, triborate,tetraborate), anionic borane and carborane clusters (e.g., B₁₀H₁₀ ²⁻andB₁₂H₁₂ ²⁻), perrhenate, permanganate, ruthenate, perruthenate, and thepolyoxometallates. The inorganic anion may, in another embodiment,include fluorine atoms. Some examples of such anions include fluoride,bifluoride (HF₂ ⁻), hexafluorophosphate (PF₆ ⁻), fluorophosphate(PO₃F²⁻), tetrafluoroborate (BF₄—), aluminum fluorides (e.g., AlF₄ ⁻),hexafluoroarsenate (AsF₆ ⁻), and hexafluoroantimonate (SbF₆ ⁻).

In another embodiment, the anion (X⁻) of the ionic liquid iscarbon-containing (i.e., organic). The organic anion may, in oneembodiment, lack fluorine atoms. Some examples of such anions includecarbonate, bicarbonate, the carboxylates (e.g., formate, acetate,propionate, butyrate, valerate, lactate, pyruvate, oxalate, malonate,glutarate, adipate, decanoate, salicylate, ibuprofenate, and the like),the sulfonates (e.g., CH₃SO₃ ⁻, CH₃CH₂SO₃ ⁻, CH₃(CH₂)₂SO₃ ⁻,benzenesulfonate, toluenesulfonate, dodecylbenzenesulfonate, docusate,and the like), the alkoxides (e.g., methoxide, ethoxide, isopropoxide,phenoxide, and glycolate), the amides (e.g., dimethylamide anddiisopropylamide), diketonates (e.g., acetylacetonate), theorganoborates (e.g., BR₁R₂R₃R₄ ⁻, wherein R₁, R₂, R₃, R₄ are typicallyhydrocarbon groups containing 1 to 6 carbon atoms), the alkylsulfates(e.g., diethylsulfate), alkylphosphates (e.g., ethylphosphate ordiethylphosphate), and the phosphinates (e.g.,bis-(2,4,4-trimethylpentyl)phosphinate). The organic anion may, inanother embodiment, include fluorine atoms. Some examples of such anionsinclude the fluorosulfonates (e.g., CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻, CF₃(CF₂)₂SO₃⁻, CHF₂CF₂SO₃ ⁻, and the like), the fluoroalkoxides (e.g., CF₃O⁻,CF₃CH₂O⁻, CF₃CF₂O⁻, and pentafluorophenolate), the fluorocarboxylates(e.g., trifluoroacetate and pentafluoropropionate), and thefluorosulfonylimides (e.g., (CF₃SO₂)₂N⁻).

In particular embodiments, the anion (X⁻) of the ionic liquid has astructure according to the following general formula:

In Formula (3) above, subscripts m and n are independently 0 or aninteger of 1 or above. Subscript p is 0 or 1, provided that when p is 0,the group —N—SO₂—(CF₂)_(n)CF₃ subtended by p is replaced with an oxideatom connected to the sulfur atom (S).

In one embodiment of Formula (3), subscript p is 1, so that Formula (3)reduces to the chemical formula:

In one embodiment of Formula (3a), m and n are the same number, therebyresulting in a symmetrical anion. In another embodiment of formula (3a),m and n are not the same number, thereby resulting in an asymmetricalanion.

In a first set of embodiments of Formula (3a), m and n are independentlyat least 0 and up to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11. When m and nare both 0, the resulting anion has the formula F₃CSO₂NSO₂CF₃, i.e.,bis-(trifluoromethylsulfonyl)imide, or Tf₂N⁻. In another embodiment, mand n are not both 0. For example, in a particular embodiment, m is 0while n is a value of 1 or above (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,or 11). Some examples of such anions include F₃CSO₂NSO₂CF₂CF₃,F₃CSO₂NSO₂(CF₂)₂CF₃, F₃CSO₂NSO₂(CF₂)₃CF₃, F₃CSO₂NSO₂(CF₂)₄CF₃,F₃CSO₂NSO₂(CF₂)₅CF₃, and so on, wherein it is understood that, in theforegoing examples, the negative sign indicative of a negative charge(i.e., “−”) in the anion has been omitted for the sake of clarity.

In a second set of embodiments of Formula (3a), m and n areindependently at least 1 and up to 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11.For example, in a particular embodiment, m is 1 while n is a value of 1or above (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11). Some examples ofsuch anions include N[SO₂CF₂CF₃]₂ (i.e., “BETI⁻”),F₃CF₂CSO₂NSO₂(CF₂)₂CF₃, F₃CF₂CSO₂NSO₂(CF₂)₃CF₃, F₃CF₂CSO₂NSO₂(CF₂)₄CF₃,F₃CF₂CSO₂NSO₂(CF₂)₅CF₃, and so on.

In a third set of embodiments of Formula (3a), m and n are independentlyat least 2 and up to 3, 4, 5, 6, 7, 8, 9, 10, or 11. For example, in aparticular embodiment, m is 2 while n is a value of 2 or above (e.g., 2,3, 4, 5, 6, 7, 8, 9, 10, or 11). Some examples of such anions includeN[SO₂(CF₂)₂CF₃]₂, F₃C(F₂C)₂SO₂NSO₂(CF₂)₃CF₃, F₃C(F₂C)₂SO₂NSO₂(CF₂)₄CF₃,F₃C(F₂C)₂SO₂NSO₂(CF₂)₅CF₃, and so on.

In a fourth set of embodiments of Formula (3a), m and n areindependently at least 3 and up to 4, 5, 6, 7, 8, 9, 10, or 11. Forexample, in a particular embodiment, m is 3 while n is a value of 3 orabove (e.g., 3, 4, 5, 6, 7, 8, 9, 10, or 11). Some examples of suchanions include N[SO₂(CF₂)₃CF₃]₂, F₃C(F₂C)₃SO₂NSO₂(CF₂)₄CF₃,F₃C(F₂C)₃SO₂NSO₂(CF₂)₅CF₃, F₃C(F₂C)₃SO₂NSO₂(CF₂)₆CF₃,F₃C(F₂C)₃SO₂NSO₂(CF₂)₇CF₃, and so on.

In another embodiment of Formula (3), subscript p is 0, so that Formula(3) reduces to the chemical formula:

In different exemplary embodiments of Formula (3b), m can be 0 or above(e.g., up to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11), 1 or above (e.g., upto 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11), 2 or above (e.g., up to 3, 4, 5,6, 7, 8, 9, 10, or 11), 3 or above (e.g., up to 4, 5, 6, 7, 8, 9, 10, or11), 4 or above (e.g., up to 5, 6, 7, 8, 9, 10, or 11), 5 or above(e.g., up to 6, 7, 8, 9, 10, or 11), 6 or above (e.g., up to 7, 8, 9,10, or 11), 7 or above (e.g., up to 8, 9, 10, 11, or 12), 8 or above(e.g., up to 9, 10, 11, or 12), or 9 or above (e.g., up to 10, 11, 12,13, 14, 15, or 16). Some examples of such anions include F₃CSO₃ ⁻ (i.e.,“triflate” or “TfO⁻”), F₃CF₂CSO₃ ⁻, F₃C(F₂C)₂SO₃ ⁻, F₃C(F₂C)₃SO₃ ⁻(i.e., “nonaflate” or “NfO⁻”), F₃C(F₂C)₄SO₃ ⁻, F₃C(F₂C)₅SO₃ ⁻,F₃C(F₂C)₆SO₃ ⁻, F₃C(F₂C)₇SO₃ ⁻, F₃C(F₂C)₈SO₃ ⁻, F₃C(F₂C)₉SO₃ ⁻,F₃C(F₂C)₁₀SO₃ ⁻, F₃C(F₂C)₁₁SO₃ ⁻, and so on.

The protic ionic liquids according to Formula (1) can be synthesized bymethods well known in the art, as evidenced by, for example, H. Luo etal., Separation Science and Technology, 45: 1679-1688, 2010, thecontents of which are herein incorporated by reference in theirentirety. In brief, an organic amide (such as in accordance with Formula2) can be reacted with a mineral acid, such as hydrochloric acid, toprotonate the amide, followed by anion exchange of the protonated amidesalt with an anion of interest, such as NTf₂ ⁻ or BETI⁻, by reaction ofthe protonated amide salt with an anion salt (e.g., LiNTf₂ or LiBETI),along with removal of salt byproduct (e.g., LiCl).

After mixing of the protic ionic liquid according to Formula (1) withthe rare earth-containing substance, a reaction occurs between theprotic ionic liquid and one or more of the rare earth elements toproduce a complex containing at least the one or more rare earthelements, the anion (X⁻), and the conjugate base of the cationic(amidium) portion of the protic ionic liquid of Formula (1). Theconjugate base of the amidium species is the amide compound resultingfrom deprotonation of the amidium species. Thus, the conjugate base ofthe amidium species in Formula (1) has the following formula:

The rare earth composition containing at least the one or more rareearth elements (RE), the anion (X⁻), and amide species of Formula (2)can be conveniently expressed by the formula (RE)(amide)_(y)X_(z),wherein the foregoing composition becomes at least partially dissolvedin the protic ionic liquid (or solvent in which the protic ionic liquidis dissolved) during the mixing process. In the rare earth composition,X is equivalent to X⁻ shown in Formula (1). The subscript y is generally2-6 (or, e.g., 2-5, 3-5, or 3-6), depending on the rare earth element.The subscript z is a number that serves to charge balance the totalpositive charge of the at least one rare earth metal (RE). When RErepresents a single rare earth element or a combination of rare earthelements having the same oxidation state, and when X is a monovalentanion, then z is an integer equivalent to the oxidation state of the atleast one rare earth element RE, as in the sub-formula(RE)(amide)_(y)X₃, in the case where RE represents a rare earth elementhaving a +3 oxidation state and X is a monovalent anion. In a case whereRE represents a rare earth element having a +3 oxidation state and X isa divalent anion, the rare earth composition may be expressed as(RE)₂(amide)_(y)X₃, in which case the subscript y may also requireadjustment in order to reflect the number of amide groups for two ormore rare earth elements. The subscript z may also be a fractionalnumber in the event that RE represents at least two rare earth elementsdiffering in oxidation state. If, for example, RE represents two rareearth elements having oxidation states of +3 (e.g., La) and +4 (e.g.,Ce) at 1:1 molar ratio, and if X is a monovalent anion, the compositionmay be expressed by the sub-formula (RE)(amide)_(y)X_(3.5). The rareearth composition may or may not include additional metallic species(e.g., one or more alkali or alkaline earth species), one or moreneutral molecules, or one or more anionic species.

In another set of embodiments, the protic ionic liquid contains aprotonated phosphine oxide species. These types of protic ionic liquidscan be expressed by the following formula:

In the above Formula (4), R⁴, R⁵, and R⁶ are independently selected fromhydrocarbon groups (R), as described above, containing at least 1 and upto 12 carbon atoms and/or selected from alkoxide groups —OR. In someembodiments, R⁴, R⁵, and R⁶ are all the same or different hydrocarbongroups (R). In other embodiments, R⁴, R⁵, and R⁶ are all the same ordifferent alkoxide groups (—OR). In yet other embodiments, R⁴, R⁵, andR⁶ are a mix of hydrocarbon groups and alkoxide groups. X⁻ is an anionicspecies, as described above.

The protic ionic liquids according to Formula (4) may be synthesizedanalogously to protic ionic liquids of Formula (1) by, for example,protonating a phosphine oxide-containing compound, such as according toFormula (5), with a mineral acid (e.g., hydrochloric acid), followed byanion exchange of the protonated phosphine oxide-containing salt with ananion of interest, such as NTf₂ ⁻ or BETI⁻, by reaction of theprotonated phosphine oxide-containing salt with an anion salt (e.g.,LiNTf₂ or LiBETI), along with removal of salt byproduct (e.g., LiCl).

The protic ionic liquid according to Formula (4) is mixed with the rareearth-containing substance in accordance with the mixing processdescribed above for the protic ionic liquid according to Formula (1). Asin the case of the amidium ionic liquid of Formula (1), the mixingprocess using the protic ionic liquid of Formula (4) results in asolution containing a rare earth complex that contains at least one ormore rare earth elements derived from the rare earth-containingsubstance along with anions (X⁻) and the deprotonated form (i.e.,conjugate base) of the cationic species in Formula (4). The deprotonatedform (i.e., conjugate base) of the cationic species in Formula (4) isherein referred to as a “phos” group, which has the following formula:

Thus, the composition resulting from reaction between the protic ionicliquid of Formula (4) and one or more rare earth elements can beconveniently expressed by the formula (RE)(phos)_(y)X_(z), wherein theforegoing composition becomes at least partially dissolved in the proticionic liquid during the mixing process. In the rare earth composition, Xis equivalent to X⁻ shown in Formula (4). The subscript y is generally2-6 (or, e.g., 2-5, 3-5, or 3-6), depending on the rare earth element.As discussed above, the subscript z is a number that serves to chargebalance the total positive charge of the at least one rare earth metal(RE). All of the examples provided above for compositions containing theneutral amide according to Formula (2) apply herein analogously tocompositions containing the neutral phosphine oxide according to Formula(5). Thus, depending on the oxidation state and number of RE elements,and the valency of X, the composition containing phosphine oxideaccording to Formula (5) may be expressed as, for example,(RE)(phos)_(y)X₃, (RE)₂(phos)_(y)X₃, or (RE)(phos)_(y)X_(3.5).

In some embodiments, the protic ionic liquid according to Formula (1) or(4) is dissolved in a solvent (e.g., an aqueous or non-aqueous solventdifferent from the protic ionic liquid) at the time the protic ionicliquid is contacted with and mixed with the rare earth-containingsubstance. The protic ionic liquid may be dissolved in the solventbefore or during the mixing process with the rare earth-containingsubstance. The solvent should not be reactive with the protic ionicliquid, such as by not being capable of deprotonating the protonatedcation of the protic ionic liquid. In the method, the protic ionicliquid serves to facilitate dissolution of one or more of the rare earthelements derived from the rare earth-containing substance into thesolvent. The protic ionic liquid does this by forming a complex with theone or more rare earth elements, as discussed above, wherein the rareearth-containing complex is at least partially soluble in the solvent.

In one set of embodiments, the solvent in which the protic ionic liquidis dissolved is aqueous-based. The aqueous-based solvent may be water ora solution of water and one or more organic or inorganic solventssoluble in water. The aqueous solution may be, for example, an aqueousalcohol solution.

In another set of embodiments, the solvent in which the protic ionicliquid is dissolved is non-aqueous, i.e., an organic solvent. Theorganic solvent may be, for example, protic or aprotic, and/or polar ornon-polar. Some examples of polar protic organic solvents include thealcohols (e.g., methanol, ethanol, isopropanol, n-butanol, t-butanol,the pentanols, hexanols, octanols, or the like) and diols (e.g.,ethylene glycol, diethylene glycol, triethylene glycol). Some examplesof polar aprotic organic solvents include the nitriles (e.g.,acetonitrile, propionitrile), sulfoxides (e.g., dimethylsulfoxide),amides (e.g., dimethylformamide, N,N-dimethylacetamide), organochlorides(e.g., methylene chloride, chloroform, 1,1,1-trichloroethane), ketones(e.g., acetone, 2-butanone), dialkylcarbonates (e.g., ethylenecarbonate, dimethylcarbonate, diethylcarbonate), organoethers (e.g.,diethyl ether, tetrahydrofuran, and dioxane), hexamethylphosphoramideacid (HMPA), N-methylpyrrolidone (NMP),1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU), propyleneglycol methyl ether acetate (PGMEA), and supercritical carbon dioxide.Some examples of non-polar organic solvents include the liquidhydrocarbons, such as a pentane, hexane, heptane, octane, pentene,hexene, heptene, octene, benzene, toluene, and xylene. In someembodiments, a combination of any of the foregoing organic solvents isused as the solvent. In other embodiments, one or more of any of theforegoing classes or specific types of organic solvents is excluded, oran organic solvent is excluded altogether in the mixing process orentire extraction process.

In yet other embodiments, the solvent in which the protic ionic liquidis dissolved is an ionic liquid, hereinafter referred to as a “secondaryionic liquid,” which is different from and non-reactive with the proticionic liquid of Formula (1) or (4). In some embodiments, the secondaryionic liquid is protic, while in other embodiments the secondary ionicliquid is non-protic. The secondary ionic liquid should have the abilityto at least partially dissolve the rare earth-containing complexresulting from reaction of the protic ionic liquid of Formula (1) or (4)with the one or more rare earth elements (RE).

The secondary ionic liquid can belong to any of the numerous classes ofionic liquids known in the art, provided it is non-reactive with theprotic ionic liquid of Formula (1) or (4) and has the ability to atleast partially dissolve the rare earth-containing complex resultingfrom reaction of the protic ionic liquid of Formula (1) or (4) with theone or more rare earth elements (RE). The secondary ionic liquid can beconveniently described by the formula (Y⁺)(X⁻), wherein Y⁺ is a cationiccomponent of the ionic liquid and X⁻ is an anionic component of theionic liquid. The formula (Y⁺)(X⁻) is meant to encompass a cationiccomponent (Y⁺) having any valency of positive charge, and an anioniccomponent (X⁻) having any valency of negative charge, provided that thecharge contributions from the cationic portion and anionic portion arecounterbalanced in order for charge neutrality to be preserved in theionic liquid molecule. More specifically, the formula (Y⁺)(X⁻) is meantto encompass the more generic formula (Y^(+a))_(w)(X^(−b))_(x), whereinthe variables a and b are, independently, non-zero integers, and thesubscript variables w and x are, independently, non-zero integers, suchthat a·w=b·x (wherein the period placed between variables indicatesmultiplication of the variables). The foregoing generic formulaencompasses numerous possible sub-formulas, such as, for example,(Y⁺)(X⁻), (Y⁺²)(X⁻)₂, (Y⁺)₂(X⁻²), (Y⁺²)₂(X⁻²)₂, (Y⁺³)(X⁻)₃, (Y⁺)₃(X⁻³),(Y⁺³)₂(X⁻²)₃, and (Y⁺²)₃(X⁻³)₂.

In some embodiments, the cationic group Y⁺ of the secondary ionic liquidhas the generic formula:

In Formula (6), Z is either N or P, and R^(1a), R^(2a), R^(3a), andR^(4a) are each independently selected from hydrogen atom andhydrocarbon groups (R) having at least one and up to twenty carbon atomsand optionally substituted with one or more heteroatoms selected fromfluorine, nitrogen, oxygen, and sulfur, as described above forhydrocarbon groups R, provided that at least one of R^(1a), R^(2a),R^(3a), and R^(4a) is a hydrocarbon group R when Z is N, and providedthat R^(1a), R^(2a), R^(3a), and R^(4a) are all hydrocarbon groups whenZ is P. In particular embodiments, one, two, three, or all of R¹, R²,R³, and R⁴ are selected from straight-chained or branched alkyl and/oralkenyl groups having at least 1, 2, 3, or 4 and up to 5, 6, 7, 8, 9,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms. In otherembodiments, one, two, three, or all of R^(1a), R^(2a), R^(3a), andR^(4a) are selected from saturated or unsaturated cyclic hydrocarbongroups, which may be carbocyclic (e.g., cycloalkyl or aryl) orheterocyclic (e.g., heterocycloalkyl or heteroaryl).

In some embodiments of Formula (6), Z is N, which corresponds toammonium species having the following formula:

In Formula (6a), R^(1a), R^(2a), R^(3a), and R^(4a) are eachindependently selected from hydrogen atom and hydrocarbon groups (R)having at least one and up to twenty carbon atoms and optionallysubstituted with one or more heteroatoms selected from fluorine,nitrogen, oxygen, and sulfur, as described above for hydrocarbon groupsR, provided that at least one of R^(1a), R^(2a), R^(3a), and R^(4a) is ahydrocarbon group R.

In some embodiments, one, two, three, or all of R^(1a), R^(2a), R^(3a),and R^(4a) of Formula (6a) are selected from straight-chained orbranched alkyl and/or alkenyl groups having at least 1, 2, 3, or 4 andup to 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20carbon atoms, or at least 5, 6, 7, or 8 and up to 9, 10, 11, 12, 13, 14,15, 16, 17, 18, 19, or 20 carbon atoms. Some examples of such ammoniumspecies include methylammonium, ethylammonium, vinylammonium,n-propylammonium, isopropylammonium, allylammonium, n-butylammonium,isobutylammonium, n-pentylammonium, n-hexylammonium, n-heptylammonium,n-octylammonium, 2-ethylhexylammonium, n-nonylammonium, n-decylammonium,n-undecylammonium, n-dodecylammonium, dimethylammonium, diethylammonium,divinylammonium, ethylmethylammonium, dipropylammonium,methylpropylammonium, diisopropylammonium, diallylammonium,dibutylammonium, methylbutylammonium, diisobutylammonium,dipentylammonium, methylpentylammonium, dihexylammonium,diheptylammonium, dioctylammonium, di(2-ethylhexyl)ammonium,dinonylammonium, didecylammonium, didodecylammonium, trimethylammonium,dimethylethylammonium, triethylammonium, trivinylammonium,tripropylammonium, triisopropylammonium, dimethylisopropylammonium,triallylammonium, tributylammonium, triisobutylammonium,diethylisobutylammonium, ethyldiisobutylammonium, tripentylammonium,trihexylammonium, triisohexylammonium, ethyldioctylammonium,trioctylammonium, tris(isooctyl)ammonium,methylbis(2-ethylhexyl)ammonium, tris(2-ethylhexyl)ammonium,trinonylammonium, tridecylammonium, tridodecylammonium,tetramethylammonium, tetraethylammonium, tetravinylammonium,tetrapropylammonium, tetraisopropylammonium,dimethyldiisopropylammonium, tetraallylammonium, tetrabutylammonium,tetraisobutylammonium, dimethyldiisobutylammonium,diethyldiisobutylammonium, methyltriisobutylammonium,ethyltriisobutylammonium, tetrapentylammonium, tetrahexylammonium,tetraisohexylammonium, ethyltrioctylammonium, tetraoctylammonium,tetrakis(isooctyl)ammonium, methyltris(2-ethylhexyl)ammonium,ethyltris(2-ethylhexyl)ammonium, tetrakis(2-ethylhexyl)ammonium,tetranonylammonium, tetradecylammonium, and tetradodecylammonium.

In other embodiments, one, two, three, or all of R^(1a), R^(2a), R^(3a)and R^(4a) of Formula (6a) are selected from saturated or unsaturatedcyclic hydrocarbon groups, which may be carbocyclic (e.g., cycloalkyl oraryl) or heterocyclic (e.g., heterocycloalkyl or heteroaryl) and may ormay not include a hydrocarbon linker and/or one more hydrocarbonsubstituents. Some examples of such ammonium species includetrimethylcyclopentylammonium, trimethylcyclohexylammonium,trimethylphenylammonium, trimethylbenzylammonium,trimethylnaphthylammonium, triethylcyclopentylammonium,triethylcyclohexylammonium, triethylphenylammonium,triethylbenzylammonium, triisopropylcyclopentylammonium,triisopropylcyclohexylammonium, triisopropylphenylammonium,triisopropylbenzylammonium, dimethylcyclopentylammonium,dimethylcyclohexylammonium, dimethylphenylammonium,dimethylbenzylammonium, diethylcyclopentylammonium,diethylcyclohexylammonium, diethylphenylammonium, diethylbenzylammonium,diisopropylcyclopentylammonium, diisopropylcyclohexylammonium,diisopropylphenylammonium, diisopropylbenzylammonium,dimethyldicyclopentylammonium, dimethyldicyclohexylammonium,dimethyldiphenylammonium, dimethyldibenzylammonium,diethyldicyclopentylammonium, diethyldicyclohexylammonium,diethyldiphenylammonium, diethyldibenzylammonium,diisopropyldicyclopentylammonium, diisopropyldicyclohexylammonium,diisopropyldiphenylammonium, diisopropyldibenzylammonium,dihexyldiphenylammonium, dioctyldiphenylammonium,dihexyldibenzylammonium, dioctyldibenzylammonium,methyltricyclopentylammonium, methyltricyclohexylammonium,methyltriphenylammonium, methyltribenzylammonium,ethyltricyclopentylammonium, ethyltricyclohexylammonium,ethyltriphenylammonium, ethyltribenzylammonium,isopropyltricyclopentylammonium, isopropyltricyclohexylammonium,isopropyltriphenylammonium, hexyltriphenylammonium,octyltriphenylammonium, isopropyltribenzylammonium,hexyltribenzylammonium, octyltribenzylammonium,tetracyclopentylammonium, tetracyclohexylammonium, tetraphenylammonium,and tetrabenzylammonium.

In some embodiments of Formula (6), Z is P, which corresponds tophosphonium species having the following formula:

In Formula (6b), R^(1a), R^(2a), R^(3a), and R^(4a) are eachindependently selected from hydrocarbon groups (R) having at least oneand up to twenty carbon atoms and optionally substituted with one ormore heteroatoms selected from fluorine, nitrogen, oxygen, and sulfur,as described above for hydrocarbon groups R. In particular embodiments,one, two, three, or all of R^(1a), R^(2a), R^(3a), and R^(4a) areselected from straight-chained or branched alkyl and/or alkenyl groupshaving at least 1, 2, 3, or 4 and up to 5, 6, 7, 8, 9, 10, 11, 12, 13,14, 15, 16, 17, 18, 19, or 20 carbon atoms, or at least 5, 6, 7, or 8and up to 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms.In other embodiments, one, two, three, or all of R^(1a), R^(2a), R^(3a),and R^(4a) are selected from saturated or unsaturated cyclic hydrocarbongroups, which may be carbocyclic (e.g., cycloalkyl or aryl) orheterocyclic (e.g., heterocycloalkyl or heteroaryl).

In some embodiments of Formula (6b), R^(1a), R^(2a), R^(3a), and R^(4a)of Formula (6b) are selected from straight-chained or branched alkyland/or alkenyl groups having at least 1, 2, 3, or 4 and up to 5, 6, 7,8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms, or atleast 5, 6, 7, or 8 and up to 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,or 20 carbon atoms. Some examples of such phosphonium species includetetramethylphosphonium, tetraethylphosphonium, tetravinylphosphonium,tetrapropylphosphonium, tetraisopropylphosphonium,dimethyldiisopropylphosphonium, tetraallylphosphonium,tetrabutylphosphonium, tetraisobutylphosphonium,dimethyldiisobutylphosphonium, diethyldiisobutylphosphonium,methyltriisobutylphosphonium, ethyltriisobutylphosphonium,tetrapentylphosphonium, tetrahexylphosphonium,tetrakis(isohexyl)phosphonium, ethyltrioctylphosphonium,tetraoctylphosphonium, tetrakis(isooctyl)phosphonium,methyltris(2-ethylhexyl)phosphonium, ethyltris(2-ethylhexyl)phosphonium,tetrakis(2-ethylhexyl)phosphonium, trihexyldodecylphosphonium,tetranonylphosphonium, tetradecylphosphonium, andtetradodecylphosphonium.

In other embodiments, one, two, three, or all of R^(1a), R^(2a), R^(3a)and R^(4a) of Formula (6b) are selected from saturated or unsaturatedcyclic hydrocarbon groups, which may be carbocyclic (e.g., cycloalkyl oraryl) or heterocyclic (e.g., heterocycloalkyl or heteroaryl) and may ormay not include a hydrocarbon linker and/or one more hydrocarbonsubstituents. Some examples of such phosphonium species includetrimethylcyclopentylphosphonium, trimethylcyclohexylphosphonium,trimethylphenylphosphonium, trimethylbenzylphosphonium,trimethylnaphthylphosphonium, triethylcyclopentylphosphonium,triethylcyclohexylphosphonium, triethylphenylphosphonium,triethylbenzylphosphonium, triisopropylcyclopentylphosphonium,triisopropylcyclohexylphosphonium, triisopropylphenylphosphonium,triisopropylbenzylphosphonium, dimethyldicyclopentylphosphonium,dimethyldicyclohexylphosphonium, dimethyldiphenylphosphonium,dimethyldibenzylphosphonium, diethyldicyclopentylphosphonium,diethyldicyclohexylphosphonium, diethyldiphenylphosphonium,diethyldibenzylphosphonium, diisopropyldicyclopentylphosphonium,diisopropyldicyclohexylphosphonium, diisopropyldiphenylphosphonium,diisopropyldibenzylphosphonium, dihexyldiphenylphosphonium,dioctyldiphenylphosphonium, dihexyldibenzylphosphonium,dioctyldibenzylphosphonium, methyltricyclopentylphosphonium,methyltricyclohexylphosphonium, methyltriphenylphosphonium,methyltribenzylphosphonium, ethyltricyclopentylphosphonium,ethyltricyclohexylphosphonium, ethyltriphenylphosphonium,ethyltribenzylphosphonium, isopropyltricyclopentylphosphonium,isopropyltricyclohexylphosphonium, isopropyltriphenylphosphonium,hexyltriphenylphosphonium, octyltriphenylphosphonium,dodecyltriphenylphosphonium, isopropyltribenzylphosphonium,hexyltribenzylphosphonium, octyltribenzylphosphonium,tetracyclopentylphosphonium, tetracyclohexylphosphonium,tetraphenylphosphonium, and tetrabenzylphosphonium.

In the cationic species of Formula (6), two or more of R^(1a), R^(2a),R^(3a), and R^(4a) may (i.e., optionally) be combined to form one ormore cyclic groups that includes Z as a ring heteroatom. By analogy, twoor more of R^(1a), R^(2a), R^(3a), and R^(4a) of Formula (6a), or two ormore of R^(1a), R^(2a), R^(3a), and R^(4a) of Formula (6b), may (i.e.,optionally) be combined to form one or more cyclic groups that includesZ as a ring heteroatom. Thus, if R^(1a) and R^(2a) are taken as ethylgroups, R^(1a) and R^(2a) may interconnect to form a five-membered ringthat includes Z. The interconnected moiety may also contain one or moreheteroatoms as a ring heteroatom, in addition to Z. Alternatively, or inaddition to two or more of R^(1a), R^(2a), R^(3a), and R^(4a)interconnecting to form a ring, two of R^(1a), R^(2a), R^(3a), andR^(4a) may (i.e., optionally) be combined to form a group linked to Z bya double bond.

For example, R^(1a) and R^(2a) in Formula (6a) or in Formula (6b) can beinterconnected to result in cationic species having any of the followingformulas:

In the above formulas, R⁷, R⁸, R⁹, R¹⁰, R¹¹, R¹², R¹³, and R¹⁴ are eachindependently selected from hydrogen atom and hydrocarbon groups (R)having at least one and up to twenty carbon atoms and optionallysubstituted with one or more heteroatoms selected from fluorine,nitrogen, oxygen, and sulfur, as described above for hydrocarbon groupsR, provided that R⁹, R¹⁰, R¹³, and R¹⁴ of the cyclic phosphonium speciesare all hydrocarbon groups R. Moreover, any of the groups, above, thatare not shown as interconnected, may either interconnect with each otherto make a second ring that includes N or P, or may interconnect with theexisting ring to form a bicyclic structure. Some examples of suchstructures include:

The interconnection of R^(1a) and R^(2a) in Formula (6a) or in Formula(6b) can also include one or more heteroatoms. Some examples of suchcationic species include:

In the above structures, R⁷, R⁸, R¹¹, and R¹² are as defined above. Thegroup R′ bound to the additional one or more hydrogen atoms isindependently selected from hydrogen atom and any of the hydrocarbongroups (R) described above, particularly straight-chained or branchedalkyl and/or alkenyl groups having 1, 2, 3, 4, 5, or 6 carbon atoms.Although the above structures show the presence of one or more ringnitrogen atoms, the cyclic cation may also include a ring oxygen atom,such as in a morpholinium-based ionic liquid.

The interconnection of R^(1a) and R^(2a) in Formula (6a) or Formula (6b)can also be accompanied by one or more double bonds in the ringcontaining N or P. If one of the double bonds is connected to N or P inthe ring, then one of R^(3a) and R^(4a) in Formula (6a) or in Formula(6b) participates to make a double bond in the ring. If none of thedouble bonds are connected to N or P in the ring, then R^(3a) and R^(4a)in Formula (6a) or R^(3b) and R^(4b) in Formula (6b) are not required toparticipate in making a double bond in the ring, i.e., the double bondoriginated in this case from one of R^(1a) and R^(2a) in Formula (6a) orone of R^(1b) and R^(2b) in Formula (6b). As above, the interconnectionmay or may not also include one or more heteroatoms. Some examples ofsuch cationic species include:

In particular embodiments, the secondary ionic liquid possesses animidazolium species as the cationic Y⁺ species. The imidazolium-basedionic liquid may have a structure of the general formula:

In Formula (7) above, R^(1b), R^(2b) and R^(3b) are each independently asaturated or unsaturated, straight-chained, branched, or cyclichydrocarbon group (R), as described above, particularly thosehydrocarbon groups containing at least 1, 2, or 3 and up to 4, 5, 6, 7,8, 9, 10, 11, or 12 carbon atoms, except that R^(3b) is often a hydrogenatom instead of a hydrocarbon. X⁻ is an anion, as also described above.In some embodiments, R^(1b) and R^(2b), or R^(1b) and R^(3b), or R^(2b)and R^(3b) are different in structure or number of carbon atoms, whereasin other embodiments, R^(1b) and R^(2b), or R^(1b) and R^(3b), or R^(2b)and R^(3b) are the same either in structure or number of carbon atoms.In different embodiments, R^(1b), R^(2b) and R^(3b) each independentlyhave a minimum of at least one, two, three, four, five, six, seven, oreight carbon atoms. In other embodiments, R^(1b), R^(2b) and R^(3b) eachindependently have a maximum of two, three, four, five, six, seven,eight, nine, ten, eleven, twelve, thirteen, fourteen, fifteen, sixteen,seventeen, or eighteen carbon atoms. In other embodiments, R^(1b),R^(2b) and R^(3b) independently have a number of carbon atoms within arange of carbon atoms bounded by any of the exemplary minimum andmaximum carbon numbers provided above. As the double bonds shown inFormula (7) are generally delocalized, other structurally equivalentdepictions may be possible for the imidazolium ring.

Some general examples of secondary ionic liquids according to Formula(7) include 1,3-dimethylimidazolium⁺X⁻, 1,2,3-trimethylimidazolium⁺X⁻,2-ethyl-1,3-dimethylimidazolium⁺X⁻,2-n-propyl-1,3-dimethylimidazolium⁺X⁻,2-n-butyl-1,3-dimethylimidazolium⁺X⁻,1-ethyl-2,3-dimethylimidazolium⁺X⁻,1-n-propyl-2,3-dimethylimidazolium⁺X⁻,1-n-butyl-2,3-dimethylimidazolium⁺X⁻, 1-methyl-3-ethylimidazolium⁺X⁻,1-methyl-3-n-propylimidazolium⁺X⁻, 1-methyl-3-isopropylimidazolium⁺X⁻,1-butyl-3-methylimidazolium⁺X⁻ (i.e., BMIM⁺X⁻),1-isobutyl-3-methylimidazolium⁺X⁻, 1,3-diethylimidazolium⁺X⁻,1-ethyl-3-n-propylimidazolium⁺X⁻, 1-ethyl-3-isopropylimidazolium⁺X⁻,1-ethyl-3-n-butylimidazolium⁺X⁻, 1-ethyl-3-isobutylimidazolium⁺X⁻,1-ethyl-3-sec-butylimidazolium⁺X⁻, 1-ethyl-3-t-butylimidazolium⁺X⁻,1,3-di-n-propylimidazolium⁺X⁻, 1-n-propyl-3-isopropylimidazolium⁺X⁻,1-n-propyl-3-n-butylimidazolium⁺X⁻, 1-n-propyl-3-isobutylimidazolium⁺X⁻,1-n-propyl-3-sec-butylimidazolium⁺X⁻,1-n-propyl-3-t-butylimidazolium⁺X⁻, 1,3-diisopropylimidazolium⁺X⁻,1-isopropyl-3-n-butylimidazolium⁺X⁻,1-isopropyl-3-isobutylimidazolium⁺X⁻,1-isopropyl-3-sec-butylimidazolium⁺X⁻,1-isopropyl-3-t-butylimidazolium⁺X⁻, 1,3-di-n-butylimidazolium⁺X⁻,1-n-butyl-3-isobutylimidazolium⁺X⁻, 1-n-butyl-3-sec-butylimidazolium⁺X⁻,1-n-butyl-3-t-butylimidazolium⁺X⁻, 1,3-diisobutylimidazolium⁺X⁻,1-isobutyl-3-sec-butylimidazolium⁺X⁻,1-isobutyl-3-t-butylimidazolium⁺X⁻, 1,3-di-sec-butylimidazolium⁺X⁻,1-sec-butyl-3-t-butylimidazolium⁺X⁻, 1,3-di-t-butylimidazolium⁺X⁻,1-methyl-3-pentylimidazolium⁺X⁻, 1-methyl-3-hexylimidazolium⁺X⁻,1-methyl-3-heptylimidazolium⁺X⁻, 1-methyl-3-octylimidazolium⁺X⁻,1-methyl-3-decylimidazolium⁺X⁻, 1-methyl-3-dodecylimidazolium⁺X⁻,1-methyl-3-tetradecylimidazolium⁺X⁻, 1-methyl-3-hexadecylimidazolium⁺X⁻,1-methyl-3-octadecylimidazolium⁺X⁻,1-(2-hydroxyethyl)-3-methylimidazolium⁺X⁻, and1-allyl-3-methylimidazolium⁺X⁻.

In Formula (7), one or both of the hydrogen atoms at the 4- and5-positions may also be substituted with a group, such as a hydrocarbongroup, such as any of the hydrocarbon groups described above, analkoxide group (—OR), hydroxy group (OH), amino group (—NH₂, —NHR, or—NR₂), carboxamide group (—C(O)NR₂ wherein one or both R groups can bereplaced with H), and/or halogen atom (e.g., F, Cl, Br, or I atom),wherein the R groups may be the same or different and may or may not beinterconnected to form a ring. For example, one or both of the 4- and5-positions of the imidazole ring may be substituted with a methyl,ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, or t-butylgroup. R³ at the 2-position may also be selected from any of theforegoing groups provided for the 4- and 5-positions. Moreover, any oneor more of R¹, R² and R³ may or may not also include an imidazole orimidazolium ring, which therefore may result in a bi-imidazolium,tri-imidazolium, or tetra-imidazolium cationic portion.

In some embodiments of Formula (7), R¹ and R³, or R² and R³ areinterconnected, thereby forming an imidazolyl-containing bicyclic ringsystem. The interconnection can be saturated or unsaturated, and may ormay not include substituting groups, as described above for thehydrocarbon groups R provided above. Some examples of ionic liquidscontaining such imidazolyl-containing bicyclic ring systems includethose according to the following formulas:

In Formulas (7a) and (7b), R⁹ and R¹⁰ independently represent ahydrocarbon group, with or without heteroatom substitution, such as anyof the hydrocarbon groups (R) described above for R^(1b), R^(2b) andR^(3b) of Formula (7). In particular embodiments, R⁹ and R¹⁰ areindependently selected from methyl, ethyl, n-propyl, isopropyl, n-butyl,isobutyl, sec-butyl, t-butyl, n-pentyl, isopentyl, neopentyl, n-hexyl,isohexyl, vinyl, and allyl groups. Any of the hydrogen atoms atavailable carbon atoms in Formulas (7a) and (7b) may be substituted witha group, such as a hydrocarbon group, such as any of the hydrocarbongroups (R) described above, an alkoxide group (—OR), hydroxy group (OH),amino group (—NH₂, —NHR, or —NR₂), carboxamide group (—C(O)NR₂ whereinone or both R groups can be replaced with H), and/or halogen atom (e.g.,F, Cl, Br, or I atom), wherein the R groups may be the same or differentand may or may not be interconnected to form a ring.

In other particular embodiments, the secondary ionic liquid is anN-hydrocarbylpyridinium-based ionic liquid having a structure of thegeneral formula:

In Formula (8), R¹¹ represents a hydrocarbon group, with or withoutheteroatom substitution, such as any of the hydrocarbon groups (R)described above under Formula (7), and the anion X⁻ can be any of theanions described above. Some general examples of N-alkylpyridinium-basedionic liquids include N-methylpyridinium⁺X⁻, N-ethylpyridinium⁺X⁻,N-n-propylpyridinium⁺X⁻, N-isopropylpyridinium⁺X⁻,N-n-butylpyridinium⁺X⁻, N-isobutylpyridinium⁺X⁻,N-sec-butylpyridinium⁺X⁻, N-t-butylpyridinium⁺X⁻,N-n-pentylpyridinium⁺X⁻, N-isopentylpyridinium⁺X⁻,N-neopentylpyridinium⁺X⁻, N-n-hexylpyridinium⁺X⁻,N-n-heptylpyridinium⁺X⁻, N-n-octylpyridinium⁺X⁻, N-n-nonylpyridinium⁺X⁻,N-n-decylpyridinium⁺X⁻, N-n-undecylpyridinium⁺X⁻,N-n-dodecylpyridinium⁺X⁻, N-n-tridecylpyridinium⁺X⁻,N-n-tetradecylpyridinium⁺X⁻, N-n-pentadecylpyridinium⁺X⁻,N-n-hexadecylpyridinium⁺X⁻, N-n-heptadecylpyridinium⁺X⁻,N-n-octadecylpyridinium⁺X⁻, N-vinylpyridinium⁺X⁻, N-allylpyridinium⁺X⁻,N-phenylpyridinium⁺X⁻, N-(2-hydroxyethyl)pyridinium⁺X⁻,N-benzylpyridinium⁺X⁻, and N-phenethylpyridinium⁺X⁻.

In Formula (8), any one or more of the hydrogen atoms on the ring carbonatoms can be substituted with one or more other groups, such as ahydrocarbon group (R), alkoxide group (—OR), hydroxy group (OH), aminogroup (—NH₂, —NHR, or —NR₂), carboxamide group (—C(O)NR₂ wherein one orboth R groups can be replaced with H), and/or halogen atom (e.g., F, Cl,Br, or I atom), wherein the R groups may be the same or different andmay or may not be interconnected to form a ring. Some examples of suchionic liquids include N-methyl-4-methylpyridinium X⁻,N-ethyl-4-methylpyridinium X⁻, N-methyl-4-ethylpyridinium X⁻,N-methyl-4-isopropylpyridinium X⁻, N-isopropyl-4-methylpyridinium X⁻,and N-octyl-4-methylpyridinium X⁻. Moreover, any one or two of the ringcarbon atoms ortho, meta, or para to the shown ring nitrogen atom in thepyridinium ring may be replaced with a respective number of ringnitrogen atoms, which may be neutral or positively charged ring nitrogenatoms.

In other particular embodiments, the secondary ionic liquid is a cyclicguanidinium-based ionic liquid. The cyclic guanidinium-based ionicliquid can have any of the structures known in the art, including thosedescribed in U.S. Pat. No. 8,129,543 and M. G. Bogdanov, et al., Z.Naturforsch, 65b, pp. 37-48, 2010, the contents of which are hereinincorporated by reference in their entirety.

The cyclic guanidinium-based ionic liquid can be described by thefollowing general formula:

In Formula (9) above, R¹¹, R¹², R¹³, R¹⁴, R¹⁵, and/or R¹⁶ groupsindependently represent a hydrocarbon group, with or without heteroatomsubstitution, such as any of the hydrocarbon groups (R) described above,or a hydrogen atom, provided that at least two of R¹¹, R¹², R¹³, R¹⁴,R¹⁵, and R¹⁶ are interconnected to form a ring or a bicyclic, tricylic,or higher cyclic ring system. In some embodiments, R¹¹, R¹², R¹³, R¹⁴,R¹⁵, and/or R¹⁶ groups are independently selected from methyl, ethyl,n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, t-butyl, n-pentyl,isopentyl, neopentyl, n-hexyl, isohexyl, vinyl, and allyl groups,provided that at least two of R¹¹, R¹², R¹³, R¹⁴, R¹⁵, and R¹⁶ areinterconnected to form a ring or a bicyclic, tricyclic, or higher cyclicring system. In a first set of embodiments, R¹¹ and R¹² areinterconnected. In a second set of embodiments, R¹³ and R¹⁴, or R¹⁵ andR¹⁶, are interconnected. In a third set of embodiments, R¹¹ and R¹³, orR¹² and R¹⁵, are interconnected. In a fourth set of embodiments, R¹⁴ andR¹⁶ are interconnected. In other embodiments, any two or three of theforegoing types of interconnections are combined. The foregoingembodiments also include the possibility that all of R¹¹, R¹², R¹³, R¹⁴,R¹⁵, and R¹⁶ groups are engaged in an interconnection. The anion X⁻ canbe any anion, such as those described above.

In other particular embodiments, the secondary ionic liquid is apiperidinium-based ionic liquid having a structure of the followinggeneral formula:

In Formula (10), R¹⁷ and R¹⁸ independently represent a hydrocarbongroup, with or without heteroatom substitution, such as any of thehydrocarbon groups (R) described above, and X⁻ can be any of the anionsdescribed above. Some examples of piperidinium-based ionic liquidsinclude 1,1-dimethylpiperidinium⁺X⁻, 1-methyl-1-ethylpiperidinium⁺X⁻,1-methyl-1-propylpiperidinium⁺X⁻, 1-methyl-1-butylpiperidinium⁺X⁻,1-methyl-1-isobutylpiperidinium⁺X⁻, 1-methyl-1-pentylpiperidinium⁺X⁻,1-methyl-1-hexylpiperidinium⁺X⁻, 1-methyl-1-heptylpiperidinium⁺X⁻,1-methyl-1-octylpiperidinium⁺X⁻, 1-methyl-1-decylpiperidinium⁺X⁻,1-methyl-1-dodecylpiperidinium⁺X⁻, 1-methyl-1-tetradecylpiperidinium⁺X⁻,1-methyl-1-hexadecylpiperidinium⁺X⁻,1-methyl-1-octadecylpiperidinium⁺X⁻, 1,1-diethylpiperidinium⁺X⁻,1,1-dipropylpiperidinium⁺X⁻, 1,1-dibutylpiperidinium⁺X⁻, and1,1-diisobutylpiperidinium⁺X⁻. In some embodiments, the piperidiniumring shown in Formula (10) may have a ring carbon atom replaced with aheteroatom selected from oxygen (O), sulfur (S), and/or nitrogen (—NR—).Moreover, any of the hydrogen atoms residing on ring carbon atoms may besubstituted with one or more other groups, such as a hydrocarbon group(R), alkoxide group (—OR), hydroxy group (OH), amino group (—NH₂, —NHR,or —NR₂), carboxamide group (—C(O)NR₂ wherein one or both R groups canbe replaced with H), and/or halogen atom (e.g., F, Cl, Br, or I atom),wherein the R groups may be the same or different and may or may not beinterconnected to form a ring.

In other particular embodiments, the secondary ionic liquid is apyrrolidinium-based ionic liquid having a structure of the followinggeneral formula:

In Formula (11), R¹⁹ and R²⁰ independently represent a hydrocarbongroup, with or without heteroatom substitution, such as any of thehydrocarbon groups (R) described above, and the anion X⁻ can be any ofthe anions described above. Some examples of pyrrolidinium-based ionicliquids include 1,1-dimethylpyrrolidinium⁺X⁻,1-methyl-1-ethylpyrrolidinium⁺X⁻, 1-methyl-1-propylpyrrolidinium⁺X⁻,1-methyl-1-butylpyrrolidinium⁺X⁻, 1-methyl-1-isobutylpyrrolidinium⁺X⁻,1-methyl-1-pentylpyrrolidinium⁺X⁻, 1-methyl-1-hexylpyrrolidinium⁺X⁻,1-methyl-1-heptylpyrrolidinium⁺X⁻, 1-methyl-1-octylpyrrolidinium⁺X⁻,1-methyl-1-decylpyrrolidinium⁺X⁻, 1-methyl-1-dodecylpyrrolidinium⁺X⁻,1-methyl-1-tetradecylpyrrolidinium⁺X⁻,1-methyl-1-hexadecylpyrrolidinium⁺X⁻,1-methyl-1-octadecylpyrrolidinium⁺X⁻, 1,1-diethylpyrrolidinium⁺X⁻,1,1-dipropylpyrrolidinium⁺X⁻, 1,1-dibutylpyrrolidinium⁺X⁻, and1,1-diisobutylpyrrolidinium⁺X⁻. In some embodiments, the pyrrolidiniumring shown in Formula (11) may have a ring carbon atom replaced with aheteroatom selected from oxygen (O), sulfur (S), and/or nitrogen (—NR—).Moreover, any of the hydrogen atoms residing on ring carbon atoms may besubstituted with one or more other groups, such as a hydrocarbon group(R), alkoxide group (—OR), hydroxy group (OH), amino group (—NH₂, —NHR,or —NR₂), carboxamide group (—C(O)NR₂ wherein one or both R groups canbe replaced with H), and/or halogen atom (e.g., F, Cl, Br, or I atom),wherein the R groups may be the same or different and may or may not beinterconnected to form a ring.

In other particular embodiments, the secondary ionic liquid is asulfonium-based ionic liquid having a structure of the following generalformula:

In Formula (12), R²⁵, R²⁶, and R²⁷ independently represent a hydrocarbongroup, with or without heteroatom substitution, such as any of thehydrocarbon groups (R) described above, and the anion X⁻ can be any ofthe anions described above. Some general examples of sulfonium-basedionic liquids include trimethylsulfonium⁺X⁻, dimethylethylsulfonium⁺X⁻,diethylmethylsulfonium⁺X⁻, triethylsulfonium⁺X⁻,dimethylpropylsulfonium⁺X⁻, dipropylmethylsulfonium⁺X⁻,tripropylsulfonium⁺X⁻, dimethylbutylsulfonium⁺X⁻,dibutylmethylsulfonium⁺X⁻, tributylsulfonium⁺X⁻,dimethylhexylsulfonium⁺X⁻, dihexylmethylsulfonium⁺X⁻,trihexylsulfonium⁺X⁻, dimethyloctylsulfonium⁺X⁻,dioctylmethylsulfonium⁺X⁻, and trioctylsulfonium⁺X⁻. In some embodimentsof Formula (12), two or three of R²⁵, R²⁶, and R²⁷ are interconnected toform a sulfonium-containing ring or bicyclic ring system, as describedabove for the phosphonium cyclic systems.

In some embodiments, any of the above general classes or specific typesof secondary ionic liquids, or general classes or specific types ofcationic portions of the above secondary ionic liquids, are excludedfrom the mixing process or from the method altogether. Alternatively, insome embodiments, a mixture of two or more of the foregoing secondaryionic liquids is used.

After the mixing process has dissolved at least a portion of the atleast one rare earth element into the protic ionic liquid (andoptionally, a solvent in which the protic ionic liquid is dissolved),the resulting rare earth-containing complex can be separated from thesolution by any of the known techniques for separating ametal-containing composition from a solution. For example, the solutionmay be cooled down to a sufficiently low temperature to facilitate oreffect precipitation or crystallization of the metal complex from thesolution. The solution may be cooled down to a temperature of, forexample, 20, 15, 10, 0, −10, −20, −30° C. Alternatively, or in addition,a phase-separation facilitator may be added to the solution to effectprecipitation or crystallization of the metal complex from solution. Thephase-separation facilitator may be, for example, a sufficientlyhydrophobic molecule, or alternatively, a highly polar molecule, such asa salt, or alternatively, a complexant or chelator (e.g., EDTA or acrown ether).

In some embodiments, the pH of the rare earth-containing solution isadjusted in such a manner as to facilitate separation of the one or morerare earth elements from the solution. In different embodiments, thesolution may be adjusted to have a pH of about, at least, above, up to,or less than, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10. Theresulting precipitated rare earth-containing complex may be the same ordifferent from the originally produced complex containing the neutralamide or phosphine oxide of Formulas (2) or (5). For example, the pH ofthe metal-containing solution may be lowered by addition of an acid(e.g., a mineral acid, such as HCl, HNO₃, or H₂SO₄) to convert the amideor phosphine oxide in the metal complex to its cationic form, therebychanging the metal complex into a less soluble form, which mayprecipitate from the solution. Significantly, the foregoing method forseparating the rare earth-containing complex from solution also servesto regenerate the protic ionic liquid. In some embodiments, a complexantor chelator (such as EDTA or a crown ether) is excluded from the method.

To aid in separation of the metal complex precipitant from the solution,the solution may also be centrifuged to facilitate or effect separation.The centrifugation can be conducted at any suitable angular velocity,such as an angular velocity of about, at least, above, up to, or below1000, 2000, 5000, 10,000, 15,000, 20,000, 25,000, or 30,000, or anyangular velocity within a range bounded by any two of these values.

In the mixing process, the protic ionic liquid may or may not be admixedwith one or more surfactants. The surfactants can be included to, forexample, enhance the efficiency of extraction of the rare earthelements. In one embodiment, the one or more surfactants include anionic surfactant, which can be either an anionic, cationic, orzwitterionic surfactant. Some examples of anionic surfactants includethe fluorinated and non-fluorinated carboxylates (e.g.,perfluorooctanoates, perfluorodecanoates, perfluorotetradecanoates,octanoates, decanoates, tetradecanoates, fatty acid salts), thefluorinated and non-fluorinated sulfonates (e.g.,perfluorooctanesulfonates, perfluorodecanesulfonates, octanesulfonates,decanesulfonates, alkyl benzene sulfonate), and the fluorinated andnon-fluorinated sulfate salts (e.g., dodecyl sulfates, lauryl sulfates,sodium lauryl ether sulfate, perfluorododecyl sulfate, and other alkyland perfluoroalkyl sulfate salts). The majority of cationic surfactantscontain a positively charged nitrogen atom, such as found in thequaternary ammonium surfactants, e.g., the alkyltrimethylammonium saltswherein the alkyl group typically possesses at least four carbon atomsand up to 14, 16, 18, 20, 22, 24, or 26 carbon atoms. Some examples ofcationic surfactants include the quaternary ammonium surfactants (e.g.,cetyl trimethylammonium bromide, benzalkonium chloride, and benzethoniumchloride), the pyridinium surfactants (e.g., cetylpyridinium chloride),and the polyethoxylated amine surfactants (e.g., polyethoxylated tallowamine). Some examples of zwitterionic surfactants include the betaines(e.g., dodecyl betaine, cocamidopropyl betaine) and the glycinates. Someexamples of non-ionic surfactants include the alkyl polyethyleneoxides,alkylphenol polyethyleneoxides, copolymers of polyethyleneoxide andpolypropyleneoxide (e.g., poloxamers and poloxamines), alkylpolyglucosides (e.g., octyl glucoside, decyl maltoside), fatty alcohols,(e.g., cetyl alcohol, oleyl alcohol), fatty amides (e.g., cocamide MEA,cocamide DEA), and polysorbates (e.g., polysorbate 20, polysorbate 40,polysorbate 60, polysorbate 80).

In the mixing process, the protic ionic liquid may or may not be admixedwith one or more extractant molecules to further enhance the extractionefficiency. The extractant molecule is generally a neutral (i.e.,non-ionic) or negatively-charged molecule, typically non-polymeric andnot an ionic liquid, typically having a molecular weight of up to orless than, for example, 5000, 2000, 1000, 500, 200, 100, or 50 g/mole.Any one or more classes or specific types of extractant molecules setforth below may be excluded from the mixing process or from the rareearth extraction or recovery process altogether.

In a first set of embodiments, the extractant molecule has a structureaccording to the following general formula:

In Formula (13) above, R²⁸ and R²⁹ are independently any of theunsubstituted or substituted hydrocarbon groups (R) described above, andsubscript r can be 0 (resulting in a vicinal diketone) or precisely, atleast, above, or up to, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,or 12, thereby resulting in a linker having an equivalent number oflinking atoms. In one embodiment, the linker subtended by r is composedof only carbon atoms, as found in acetylacetate (wherein R²⁸ and R²⁹ areboth methyl groups and r is 1). In other embodiments, the linkersubtended by r includes one or more linking heteroatoms, such as —O—,—NH—, —NR—, or —S—, either by replacing a linking carbon atom or byinserting between linking carbon atoms.

In particular embodiments of Formula (13), at least one of R²⁸ and R²⁹is an amino group, such as —NR₂, —NHR, or —NH₂, wherein the R groups arethe same or different. In further particular embodiments of Formula(13), both of R²⁸ and R²⁹ are amino groups according to the followingformula:

In Formula (13a), R³⁰, R³¹, R³², and R³³ are independently H or any ofthe unsubstituted or substituted hydrocarbon groups (R) described above.In particular embodiments of Formula (10a), one, two, three, or all ofR³⁰, R³¹, R³², and R³³ are independently selected from straight-chainedor branched alkyl groups having at least two, three, four, five, six,seven, eight, nine, ten, eleven, or twelve carbon atoms, or a number ofcarbon atoms within a range of these numbers.

In further particular embodiments of Formula (13a), the extractantmolecule is a diglycolamide having a structure according to thefollowing general formula:

In particular embodiments of Formula (13b), R³⁰, R³¹, R³², and R³³ areindependently selected from methyl, ethyl, n-propyl, isopropyl, n-butyl,isobutyl, sec-butyl, t-butyl, n-pentyl, isopentyl, neopentyl, n-hexyl,isohexyl, n-heptyl, isoheptyl, n-octyl, isooctyl, n-nonyl, isononyl,n-decyl, isodecyl, n-undecyl, n-dodecyl, cyclopropyl, cyclobutyl,cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, vinyl, allyl, phenyl,benzyl, tolyl, xylyl, and naphthyl groups. In some embodiments, R³⁰,R³¹, R³², and R³³ all have the same carbon number, or are all the samegroup. In some embodiments, R³⁰ and R³¹, and/or R³² and R³³ areinterconnected to form a diglycolamide having one or twonitrogen-containing heterocyclic rings. A particular example of anextractant molecule according to Formula (13b) isN,N,N′,N′-tetraoctyldiglycolamide (TODGA), i.e., wherein R³⁰, R³¹, R³²,and R³³ are each octyl (C₈H₁₇) groups.

The linker subtended by r in Formula (13) may alternatively include orbe replaced with a linking carbocyclic or heterocyclic ring. Thecarbocyclic ring may be, for example, a cyclohexyl, cyclohexenyl,phenyl, or naphthyl ring. The heterocyclic ring may be, for example, apyridyl, pyrrolyl, piperidinyl, pyranyl, or furyl ring. When aheterocyclic ring is directly bound to the two shown carbonyl groups inFormula (13), in particular embodiments, the two carbonyl groups arebound in ortho positions of the heterocyclic ring. In particularembodiments, when the heterocyclic ring is a pyridine ring, theextractant molecule can have a structure of the following formula:

In a second set of embodiments, the extractant molecule has a phosphoricacid or phosphate structure according to the following general formula:

In Formula (14), R³⁴, R³⁵, and R³⁶ are independently H or any of theunsubstituted or substituted hydrocarbon groups (R) described above,provided that at least one of R³⁴, R³⁵, and R³⁶ is a hydrocarbon group(R). In particular embodiments of Formula (14), one, two, or all of R³⁴,R³⁵, and R³⁶ are independently selected from straight-chained orbranched alkyl groups having at least two, three, four, five, six,seven, eight, nine, ten, eleven, or twelve carbon atoms, or a number ofcarbon atoms within a range of these numbers. In a first set ofembodiments of Formula (14), one of R³⁴, R³⁵, and R³⁶ is a hydrocarbongroup (R) while two of R³⁴, R³⁵, and R³⁶ are hydrogen atoms. Someexamples of such compounds include monoethylphosphoric acid,monoisopropylphosphoric acid, mono(n-butyl)phosphoric acid,monoisobutylphosphoric acid, monoisopentylphosphoric acid,mononeopentylphosphoric acid, and mono(2-ethylhexyl)phosphoric acid(H2MEHP). In a second set of embodiments of Formula (14), two of R³⁴,R³⁵, and R³⁶ are independently selected from hydrocarbon groups (R)while one of R³⁴, R³⁵, and R³⁶ is a hydrogen atom. Some examples of suchcompounds include diethylphosphoric acid, diisopropylphosphoric acid,di(n-butyl)phosphoric acid, diisobutylphosphoric acid,diisopentylphosphoric acid, di(neopentyl)phosphoric acid,dioctylphosphoric acid, and di(2-ethylhexyl)phosphoric acid (HDEHP). Ina third set of embodiments of Formula (14), all three of R³⁴, R³⁵, andR³⁶ are independently selected from hydrocarbon groups (R). Someexamples of such compounds include triethylphosphate,triisopropylphosphate, tri(n-butyl)phosphate, triisobutylphosphate,triisopentylphosphate, tri(neopentyl)phosphate, trioctylphosphate,tricresylphosphate, dicresylphenylphosphate, andtris(2-ethylhexyl)phosphate (TEHP).

In a third set of embodiments, the extractant molecule has an amidestructure according to the following general formula:

In Formula (15), R³⁷, R³⁸, and R³⁹ are independently H or any of theunsubstituted or substituted hydrocarbon groups (R) described above,provided that at least one of R³⁷, R³⁸, and R³⁹ is a hydrocarbon group(R). Typically, at least R³⁹ is a hydrocarbon group (R), and moretypically, at least R³⁹ and at least one or both of R³⁷ and R³⁸ arehydrocarbon groups (R). In particular embodiments of Formula (15), one,two, or all of R³⁷, R³⁸, and R³⁹ are independently selected fromstraight-chained or branched alkyl groups having at least two, three,four, five, six, seven, eight, nine, ten, eleven, or twelve carbonatoms, or a number of carbon atoms within a range of these numbers. In afirst set of embodiments of Formula (15), one of R³⁷, R³⁸, and R³⁹ is ahydrocarbon group (R) while two of R³⁷, R³⁸, and R³⁹ are hydrogen atoms.In a second set of embodiments of Formula (15), two of R³⁷, R³⁸, and R³⁹are independently selected from hydrocarbon groups (R) while one of R³⁷,R³⁸, and R³⁹ is a hydrogen atom. In a third set of embodiments ofFormula (15), all three of R³⁷, R³⁸, and R³⁹ are independently selectedfrom hydrocarbon groups (R). Some examples of compounds of Formula (15)include N,N-di-(2-ethylhexyl)-3-methylbutanamide,N,N-di-(2-ethylhexyl)-2-methylpropanamide,N,N-di-(2-ethylhexyl)-2,2-dimethylpropanamide, andN,N-di-(2-ethylhexyl)-2-ethylhexanamide.

In some embodiments, the solution containing the rare earth-containingcomplex dissolved in the protic ionic liquid (and optionally, solvent)is subjected to an electroplating process that results inelectrodeposition of the one or more rare earth elements from thesolution. The general methodology and conditions for such anelectrodeposition process are well known in the art, such as describedin detail in E. Bourbos et al., ERES2014: 1^(st) European Rare EarthResources Conference (Milos) 4-7, Sep. 2014, pp. 156-162 and U.S. Pat.No. 6,306,276. Notably, the methodology and conditions known in the artfor electrodeposition of rare earth metals may require adjustment (e.g.,in pH) to prevent degradation of the particular ionic liquids used inthe above-described extraction method, and also to prevent degradationof the rare earth complex dissolved in solution so that the rare earthcomplex remains dissolved and does not precipitate from solution.

Examples have been set forth below for the purpose of illustration andto describe certain specific embodiments of the invention. However, thescope of this invention is not to be in any way limited by the examplesset forth herein.

Examples Synthesis of the Protic Ionic Liquid N,N-dimethylacetamidiumbis(trifluoromethylsulfonyl)imide, i.e., (DMAH⁺NTf₂ ⁻) (Under Formula(1) Above)

Batches of DMAH⁺NTf₂ ⁻ ionic liquid were synthesized by combining theneutralization and metathesis methodologies previously described in H.Luo et al., Separation Science and Technology, 45: 1679-1688, 2010. Atroom temperature, N,N-dimethylacetamide (4.03 g, 0.046 mol) was mixedwith a slight excess of concentrated aqueous HCl, and LiNTf₂ dissolvedin deionized water was added in an equal molar ratio to this mixture.After stirring, two phases were present with water on the top andDMAH⁺NTf₂ ⁻ on the bottom. The IL layer was separated from the water andwashed several times with deionized water to remove any LiCl still inthe IL. The resulting nearly colorless liquid was dried under vacuum at70° C. for 4 hours and characterized by thermogravimetric analysis andNMR spectroscopy. Yield: 94%. ¹H NMR (400.13 MHz, CDCl₃): δ=8.07 (br.,1H), 3.33 (s, 3H), 3.26 (s, 3H), 2.48 (s, 3H) ppm. ¹³C NMR (100.61 MHz,CDCl₃): δ=174.45 (C), 119.57 (q, JC, F=320.1 Hz, CF₃), 39.69 (CH₃),37.68 (CH₃), 17.84 (CH₃) ppm.

Synthesis of the Secondary Ionic Liquid 1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide, i.e., (BMIM⁺NTf₂ ⁻)

The ionic liquid BMIM⁺NTf₂ ⁻ was synthesized according to a modifiedprocedure reported in the literature (P. Bonhote, et al., Inorg. Chem.1996, 35, 1168-1178). Methylimidazole (20.0 g, 243.0 mmol) was allowedto react with 1-bromobutane (34.0 g, 312.0 mmol) in dry THF undernitrogen. The mixture was stirred vigorously at room temperature for 24hours and the resulting white precipitate was isolated and washedrepeatedly with ethyl acetate (3×100 mL) to remove any unreactedstarting materials. The resulting BMIM⁺Br⁻ salt was dried under vacuum(yield: 90%). This BMIM⁺Br⁻ salt (20.0 g, 105.0 mmol) was mixed withLiNTf₂ (45.0 g, 156.8 mmol) in water (20 mL) and the mixture was stirredovernight to obtain BMIM⁺NTf₂ ⁻ as a colorless liquid. The ionic liquidwas dried in vacuo at 333 K for 24 hours. ¹H NMR (400 MHz, [D₆]DMSO):δ=9.10 (s, 1H), 7.75 (s, 1H), 7.67 (s, 1H), 4.19 (q, J=7.2 Hz, 2H), 3.85(s, 3H), 1.42 (t, J=7.2 Hz, 3H) ppm.

Synthesis of Rare-Earth Bastnaesite Analogues

Bastnaesite analogues containing a single lanthanide or yttrium weresynthesized according to literature procedure (O. Janka, T. Schleid,Eur. J. Inorg. Chem. 2009, 357-362). (Y,Ln)(NO₃)₃.xH₂O solid (15 mmol)was dissolved in H₂O (250 mL) and the mixture was continuously stirred.A solution containing NaF (13 mmol) and NaHCO₃ (13 mmol) in H₂O (500 mL)was added slowly to this Ln(NO₃)₃ solution over 6 hours. The resultingsolid and supernatant were then stirred for 30 minutes, filtered through0.2 μm Millipore™ filters and washed with 18 M H₂O (3×100 mL) to removeremaining NaNO₃ and unreacted Ln(NO₃)₃, NaF, and NaHCO₃. The resultingpowder was collected and dried in an oven at 120° C.

A slight modification to synthesize bastnaesite analogues containingmultiple REs was accomplished by blending the RE(NO₃)₃.xH₂O solidspre-synthesis. Table 1, below, shows the distributions of the REs ineach material.

TABLE 1 Composition of synthetic bastnaesite analogues Rare EarthBastnaesite - Mountain Bastnaesite - 10% Heavies Element Pass Blend (%)Blend Enriched (%) La 33.8 17.2 Ce 49.6 34.4 Pr 4.1 5.2 Nd 11.2 17.2 Sm0.9 4.3 Eu 0.1 4.3 Gd 0.2 3.0 Tb 0 0.5 Dy 0 3.0 Ho 0 0.7 Er 0 2.2 Tm 00.3 Yb 0 1.5 Lu Trace 0.3 Y 0.1 5.7

Synthesis of Neodymium Bis(trifluoromethylsulfonyl)imide, i.e.,(Nd(NTf₂)₃)

Nd(NTf₂)₃ solid was prepared according to the reactionNd₂O₃+6HNTf₂→2Nd(NTf₂)₃+3H₂O. Nd₂O₃ (5.2 mmol, 1.74 g) was suspended bystirring in deionized water (5 mL) at 20° C. as 80% HNTf₂ (10.9 g) wasadded to the solution. When no Nd₂O₃ remained, the solution was heatedto 120° C. under a blanket of argon (Ar) to drive off excess HNTf₂ andwater. The resulting slightly-purple solid was titrated to determine theNd³⁺ content and identified to be Nd(NTf₂)₃.2H₂O. This solid was storedunder Ar for later use.

X-Ray Diffraction Analysis

Continuous θ-2θ scans were performed with a PANalytical® Empyreandiffractometer equipped with a Pixcel® 3D detector from nominally 15 to90° 2θ in 10 minutes using Cu—Kα radiation (l=1.5405981 Å) and anX'Celerator detector. All the scans used ½° fixed slits and 1°anti-scatter slits. A search match was conducted by using the “Jade”and/or HighScore software and the International Center for DiffractionData (ICDD) database with “HighScore Plus” software.

FTIR-ATR Spectroscopy

FTIR-ATR spectra were collected with a PerkinElmer® Frontier FTIRspectrometer equipped with a diamond ATR. Scans were performed in therange 4000-650 cm⁻¹ at a resolution of 4 cm⁻¹. The crystal was cleanedin between runs with 2-propanol.

Rare-Earth Dissolution

In this experiment, the acidic amide ionic liquid (IL)N,N-dimethylacetamidium bis-(trifluoromethylsulfonyl)imide (DMAH⁺NTf₂ ⁻)in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide(BMIM⁺NTF₂ ⁻) diluent was used to dissolve froth flotation concentratebastnaesite and synthetic bastnaesite analogues [RE(CO₃)F]. To recyclethe DMAH⁺NTF₂ ⁻ a strong mineral acid is required for reprotonation.FIG. 1 shows the structures of the cationic and anionic components ofthe ionic liquids studied in the following experiments.

A 2 mol/kg DMAH⁺NTf₂ ⁻ solution in BMIM⁺NTf₂ ⁻ was used to dissolve thebastnaesite analogues Nd₂(CO₃)₃ and Nd₂O₃. For each replicate, a 10-foldexcess of acid to RE was used to ensure completion of the reaction. Thistypically involved the contact of RE solid (0.2 mmol) with IL solution(1 g) in 4 mL borosilicate glass vials. The IL solution was heated to120° C. prior to contact with the RE solids. The suspended solids weremixed at around 1000 rpm for 1 or 24 hours, and then exposed to theatmosphere. After the contact time, these samples were centrifuged at3000 rpm for 40 minutes to ensure complete separation of remaining solidfrom the IL.

To assess the kinetics of the system on the froth flotation ofbastnaesite, the temperature was set to 45° C. with stirring at 700 rpm.Aliquots of 30 μL were taken and quenched in dichloroethane (DCE). ThisDCE phase was then contacted with a 2% HNO₃ phase in a 1:1 volume ratioand agitated for 10 minutes. Under these system conditions, the RE wasstripped into the aqueous phase at >99% and the IL remained behind inthe DCE phase. These prepared nitric acid phases were injected into theIC.

To examine the kinetics of a larger scale version of this system,bastnaesite (3 g) was dissolved in a 10-fold excess of 2 mol/kgDMAH⁺NTf₂ ⁻ in BMIM⁺NTf₂ ⁻ solution (62.3 g) at a temperature of 45° C.with stirring at 1000 rpm. Aliquots of 100 μL were taken and quenched inDCE and stripped by using 2% HNO₃. The stripped solutions were dilutedappropriately and investigated by inductively coupled plasma massspectrometry.

Determination of the Rare Earth Concentration

The resulting IL phases were titrated according to previous literatureon spectrophotometric RE determinations in aqueous media (e.g., M. A. E.Hafez, et al., Analyst, 1990, 115, 221-224). Small aliquots (10-20 μL)of the RE-containing IL phase were dissolved in 0.1 mol/L sodium acetatebuffer (5 mL; pH 5). These solutions were spiked with Xylenol Orange indistilled water. A 0.01 mol/L Na₂EDTA solution was used to titrate theRE acetate solution. The solutions changed from red to yellow toindicate that all the RE³⁺ in solution had been complexed by EDTA. Ionchromatography was used to analyze several lanthanides simultaneously byusing the following eluent mixture: 63% 160 mM oxalic acid/100 mMpotassium hydroxide/200 mM tetramethylammonium hydroxide, 4% 160 mMdiglycolic acid/190 mM potassium hydroxide, and 33% degassed deionizedwater. The flow rate was set to 1.2 mL/min with a back pressure of 1750psi. The post column reagent used was 0.5 mm 4-(2-pyridylazo)resorcinol(PAR, P/N 39672 in Met Pac PAR) post-column reagent diluent (P/N046094). Injections of 2% nitric acid containing RE/IL aliquots allowedfor baseline resolution of RE peaks (except Pr and Nd) in thechromatograms and linearity of the signal response from 50 ppb to 1 ppm.Spectrophotometric titrations were also conducted. The absorptionspectra of Nd(NTf₂)₃ in BMIM⁺NTf₂ ⁻ were collected with a 1-cm pathlength over the wavelength range 500-900 nm with a VIS-NIRspectrophotometer equipped with a tungsten-halogen source. A 0.09 mol/kgNd(NTf₂)₃ solution in BMIM⁺NTf₂ ⁻ was titrated with 6 mol/kgN,N-dimethylacetamide in BMIM⁺NTf₂ ⁻ at 120° C. The stability constantsfor the Nd³⁺/N,N-dimethylacetamide complexes were calculated bynonlinear least-square regression in HypSpec software (P. Gans, et al.,Talanta 1996, 43, 1739-1753).

Dissolution of Bastnaesite-Type Solids

A series of rare-earth carbonate fluorides (RECO₃F, RE=La, Ce, Pr, Nd,Eu, Tb, Dy, Ho, Y) were synthesized for use in these dissolutionstudies. In addition, bastnaesite analogues containing RE blends werealso synthesized (as provided in Table 1). FIG. 2 shows no selectivityduring the dissolution process for the synthesized bastnaesite at 24hours. In FIG. 2, the percentage of RE dissolved in the IL phase isdefined by the following equation:

${\% \mspace{14mu} {RE}\mspace{14mu} {dissolved}} = {\frac{\left\lbrack {RE}^{3 +} \right\rbrack_{IL}}{\left\lbrack {RE}^{3 +} \right\rbrack_{total}} \times 100}$

in which [RE³⁺]_(IL) is the concentration of the RE³⁺ metal in the ILphase, as determined through EDTA titration, and [RE³]_(total) is theconcentration of the RE³⁺ metal if all of the RECO₃F solid could bedissolved and exist in solution. The RECO₃F solids gave an average % REDissolved value of 60±6%, which is lower than anticipated. As expected,the Nd₂(CO₃)₃ and Nd₂O₃ solids were quantitatively dissolved at 24hours. An unexpected result was the slow dissolution of LaCO₃F, CeCO₃F,PrCO₃F, and NdCO₃F compared with the heavy-RE-containing RECO₃F when themedia was analyzed at 1 hour. This led to an investigation of thestructural identity of these materials.

The synthesized bastnaesite analogues were characterized by using powderX-ray diffraction (PXRD). The PXRD patterns are shown in FIG. 3, labeledas 3a-3e as follows: synthesized (3a) NdCO₃F, (3b) CeCO₃F, (3c) PrCO₃F,(3d) LaCO₃F, and (3e) natural bastnaesite (La). As shown in FIG. 3, thePXRD patterns are in agreement with the patterns in the literature forLaCO₃F and CeCO₃F (O. Janka, T. Schleid, Eur. J. Inorg. Chem. 2009,357-362; I. Oftedal, Z. Kristallogr. Kristallgeom. Kristallphys.Kristallchem., 1930, 72, 239-248; and Y. Shaohua, et al., Trans.Nonferrous Met. Soc. China 2011, 21, 2306-2310). No literature data wasfound for comparison with the PrCO₃F or NdCO₃F solids. The preparedsingle-RECO₃F solids all appeared to crystallize as a bastnaesite-typestructure with the hexagonal space group P62c for RE=La, Ce, Pr, and Nd.All four of the light lanthanides (La, Ce, Pr, Nd) show a similardiffraction pattern to that of natural bastnaesite. For the syntheticRECO₃F solids with RE=Eu, Tb, Dy, Ho, and Y, the XRD patterns showedamorphous structures. Attempts to anneal these heavier RE solids, so arecognizable XRD pattern could be collected, yielded a black powder. ThePXRD pattern of the black powder did not match that of RECO₃F and isbelieved to be REOF.

Because the PXRD patterns of the heavier RE solids showed amorphousstructures, FTIR-ATR spectroscopy was used to qualitatively determinethe coordination of the CO₃ ²⁻ anion in the RECO₃F matrix. FIG. 4Adisplays the IR spectra of a series of synthesized bastnaesite analoguescontaining single rare earths and blends of rare earths, and FIG. 4Bcompares the spectra of the heavier TbCO₃F and YCO₃F with those ofTb₂(CO₃)₃ and Y₂(CO₃)₃. The designations in FIGS. 4A and 4B are asfollows: (4a.1) Bastnaesite-Mtn Pass Blend, (4a.2) Bastnaesite-10%Heavies Blend, (4a.3) LaCO₃F, (4a.4) CeCO₃F, (4a.5) PrCO₃F, (4a.6)NdCO₃F, (4a.7) TbCO₃F, (4a.8) DyCO₃F, (4a.9) HoCO₃F, (4a.10) YCO₃F,(4b.1) TbCO₃F, (4b.2) YCO₃F, (4b.3) Tb₂(CO₃)₃-xH₂O, and (4b.4)Y₂(CO₃)₃-xH₂O. Although the heavy-RECO₃F solids do not appear to beexplicitly “bastnaesite-like”, it can be stated that they are not REcarbonates either. A clear change in the light- and heavy-RECO₃Fsynthetic minerals occurs between Nd and Tb. The amorphous nature of theheavy-RE minerals coupled with the change in CO stretching at around1500 cm⁻¹ between NdCO₃F and TbCO₃F indicates a change in the overallstructure of the bastnaesite analogues.

Role of Carbonate Chemistry in Dissolution

The dissolution of Nd₂(CO₃)₃ was significantly slower than that of Nd₂O₃and the contrast between the dissolution of carbonate and oxide speciesassists in providing a further understanding of the chemistryresponsible for the dissolution of the bastnaesite structure inBMIM⁺NTf₂ ⁻. As the dissolution of the Nd₂(CO₃)₃ and Nd₂O₃ progresses atseemingly different rates, this raised the question of the role of CO₃²⁻ in the interaction of DMAH⁺ with RECO₃F. To further understand therole of CO₃ ²⁻ in these systems, two tests were carried out on HoCO₃F,which had already been shown to have a faster dissolution within 1 hour.The first test was to run the same experiment under the same conditions,but instead cap the vial and create a closed system. The second test wasto flush a stream of CO₂(g) over the IL solution as the reactionproceeded. In both cases, the dissolution reaction was suppressed andonly 8.2±4% of the HoCO₃F dissolved within 1 hour instead of the 53±5%(FIG. 2) that dissolved in the open system. This appears to be inagreement with the observation of gas evolution through the IL; chemicalintuition hints that this is most likely CO₂(g). Therefore, it appearsthat the Nd₂O₃ is dissolved by the DMAH⁺NTf₂ ⁻ according to thefollowing reaction:

Nd₂O₃+6DMAH⁺NTf ₂ ⁻→2Nd(DMA)₃(NTf ₂)₃+3H₂O

whereas, most likely, Nd₂(CO₃)₃ is dissolved in two reactions:

Nd₂(CO₃)₃+6DMAH⁺NTf ₂ ⁻→2Nd(DMA)₃(NTf ₂)₃+3H₂CO₃

3H₂CO₃→3CO_(2(g))+3H₂O

As a result of carbonic acid disproportionation in the IL, it seems thatthis reaction slows down the dissolution process as the system waits toexpel CO₂(g).

Rare-Earth Fluoride Precipitation

After each RECO₃F dissolution experiment, solid remained in the bottomof each vial. This solid was analyzed by PXRD and determined to beREF₃(s), as shown by the PXRD patterns in FIG. 5, with the patternsdesignated as (5a) HoF₃ with literature comparison (5b) and NdCO₃Fpost-dissolution as (5c) NdF₃ and the literature comparison (5d) after 5hours. A light (Nd) and heavy (Ho) RECO₃F were chosen for analysis,post-dissolution.

In addition to the formation of REF₃ post-dissolution, the RE³⁺ in theIL phase was also analyzed for varying total amounts of RECO₃F. As shownby the graph in FIG. 6, the number of mmol of RE³⁺ in the IL phase wasplotted versus the total number of mmol of RECO₃F(s) in the system andyielded slopes of 0.6±0.06, 0.5±0.04, and 0.5±0.07 for Nd, Dy, and Ho,respectively, which shows that roughly ⅔ of the RE exists in the ILphase after dissolution. The observation of REF₃(s) coupled with theslope analysis for one light (Nd) and two heavy (Dy, Ho) REs suggeststhat the chemistry of dissolution of RECO₃F is similar across therare-earth series.

Having established the above information, the following points can beconcluded:

1) Gas evolves as a result of DMAH⁺NTf₂ ⁻ contact with RECO₃F,Nd₂(CO₃)₃, and Nd₂O₃.

2) An open system allows the dissolution process to proceed whereas aclosed system or a system flushed with CO₂(g) suppresses the dissolutionof RECO₃F.

3) The PXRD pattern of the resulting RE solid after RECO₃F is dissolvedby DMAH⁺NTf₂ ⁻ indicates that REF₃ is a product of dissolution.

4) Only ⅔ of the RE³⁺ in the system post-dissolution exists in the ILphase. The other ⅓ exists as REF₃ solid.

5) Acid dissociation constants in aqueous media (μ=0.0 M, 25° C.) for HF(pK_(a)=3.17) and H₂CO₃ (pK_(a,1)=6.35, pK_(a,2)=10.34) suggest thatcarbonic acid forms preferentially to hydrofluoric acid.

From these considerations, the reaction likely proceeds according to thescheme shown in FIG. 7. As shown in the scheme of FIG. 7, the surface ofthe RECO₃F reacts with the H⁺ of DMAH⁺ to form soluble RE(DMA)₂(NTf₂)₂Fand carbonic acid (H₂CO₃). These two species react by two differentpathways. RE(DMA)₂(NTf₂)₂F reacts with another RE(DMA)₂(NTf₂)₂F to formsoluble RE(DMA)₂(NTf₂)₃ and insoluble REF₃, and H₂CO₃ produces carbondioxide gas and water, both of which leave the IL at 120° C. Althoughthere remains unconsumed H⁺ in the IL phase, in the form of DMAH⁺NTf₂ ⁻(pKa=−0.19 at 25° C. in sulfuric acid), it seems that the remainder ofH⁺ does not compete with RE³⁺ for F⁻, and REF₃(s) precipitates out ofsolution as a result. The following dissolution reactions are proposed:

3RECO₃F(s)+6DMAH⁺NTf ₂ ⁻→3RE(DMA)₂(NTf ₂)₂F+3H₂CO₃

3RE(DMA)₂(NTf ₂)₂F→REF₃(s)+2RE(DMA)₃(NTf ₂)₃

3H₂CO₃→3CO_(2(g))+3H₂O

3RECO₃F(s)+6DMAH⁺NTf ₂ ⁻→2RE(DMA)₃(NTf ₂)₃+REF_(3(s))+3CO_(2(g))+3H₂O

Solution Chemistry of Nd³⁺ in BMIM⁺NTf₂ ⁻ Secondary IL

As the RECO₃F mineral is dissolved by DMAH⁺NTf₂ ⁻, the RE³⁺ species thatis liberated from the solid phase becomes solvated by the constituentsof the IL phase. In this system, DMA, DMAH⁺NTf₂ ⁻ and BMIM⁺NTf₂ ⁻ arepresent in sufficient concentrations to accomplish the task of RE³⁺solvation. The following equilibrium was considered for ascertainingNd³⁺ speciation in BMIM⁺NTf₂ ⁻:

Nd³⁺ +nDMA⇄Nd(DMA)_(n) ³⁺(n=1-5)

FIG. 8A shows absorbance spectra and FIG. 8B shows speciation of Nd³⁺ inBMIM⁺NTf₂ ⁻ at 120° C. The following species are present: 0: Nd³⁺; 1:Nd(DMA)³⁺; 2: Nd(DMA)₂ ³⁺; 3: Nd(DMA)₃ ³⁺; 4: Nd(DMA)₄ ³⁺; 5: Nd(DMA)₅³⁺. Experimental Nd³⁺/DMA ratios range from 1:0 to 1:7.25. Note: the Ndspectra are shifted in order to display the features of each. The finalNd³⁺/DMA species was found to be a 1:5 complex in BMIM⁺NTf₂ ⁻ at 120° C.as no further spectral features evolved as a result of increasing theNd³⁺/DMA ratio beyond 1:7. FIG. 8A shows the hypersensitive 4F_(5/2),2H_(9/2)→4I_(9/2) transition of the Nd³⁺ ion (D. G. Karraker, Inorg.Chem. 1968, 7, 473) as it is complexed at increasing concentration ofN,N-dimethylacetamide, the conjugate base of the acidic ionic liquidDMAH⁺NTf₂ ⁻. FIG. 8B displays the speciation of the metal as a result ofthe best-fit stability constants for these Nd³⁺/DMA complexes determinedas log β₁₀₁=2.1±0.05, log β₁₀₂=6.7±0.05, log β₁₀₃=8.76±0.05, logβ₁₀₄=10.72±0.05, and log β₁₀₅=9.70±0.05. For the dissolution processunder the experimental conditions stated earlier, the average number ofligands n bound to Nd³⁺ is 2.91; however, this does not account for thenumber of potential DMA ligands that may bind with Nd³⁺ as a result ofpartially dissociated DMAH⁺, so the number may be greater.

In addition to the DMA titration of Nd³⁺, DMAH⁺NTf₂ ⁻ and HNTf₂ werealso used as titrants. HNTf₂ showed no interaction with Nd³⁺, whereasDMAH⁺NTf₂ ⁻ showed some interaction with Nd³⁺ through minor changes inthe spectral features of Nd³⁺, but no fit could be made of the data.However, it is not clear whether the DMAH⁺ ion interacts with Nd³⁺ toform the Nd(DMAH)⁴⁺ complex or the acid dissociates to a reasonableextent allowing for Nd³⁺ and DMA to interact. If any mode of interactionoccurs, it is most likely the latter.

Dissolution of Bastnaesite and Synthetic Bastnaesite RE Blends

The surprising result of a seemingly kinetically limited dissolution oflight-RECO₃F compared with heavy-RECO₃F (FIG. 2) prompted an explorationof the dissolution process of natural bastnaesite. A froth flotationbastnaesite product (enriched in RE) was obtained, and two bastnaesitesolids were synthesized as well. The compositions of these synthesizedmaterials are presented in Table 1 above. The dissolution behavior ofthe synthesized bastnaesite materials containing the entire range of REwas not particularly intriguing as these solids dissolved in nearly thesame ratios as their chemical make-up.

As was observed with the single-RE bastnaesite solids, the heavy REs innatural bastnaesite also dissolved more quickly than the light REs. Toobserve this phenomenon, the temperature of the system was lowered to45° C. with a stirring rate of 700 rpm. The ratio of the total amount ofheavy- to light-REs was calculated to determine the enrichment effectover time. The [RE] in the IL was determined by ion chromatography (IC).Each RE was calibrated by IC to be in the range 50 ppb-1 ppm by usingthe following expression:

${{Fraction}\mspace{14mu} \frac{Heavy}{Light}} = \frac{\left\lbrack {RE}^{3 +} \right\rbrack_{{Sm} - {Lu}}}{\left\lbrack {RE}^{3 +} \right\rbrack_{{LA} - {Nd}}}$

in which [RE³⁺]_(Sm-Lu) is the summation to the total heavy-rare-earthconcentration in the IL phase for RE=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,Lu, and Y, and [RE³⁺]_(La-Nd) is the summation to the totallight-rare-earth concentration in the IL phase for RE=La, Ce, Pr, andNd.

FIG. 9 shows the enrichment as a function of time. Although enrichmentof the heavy REs may occur near the beginning of the dissolutionprocess, over the course of time it appears that a combination of eventsmay occur such that the enrichment then decreases again. These eventsare 1) the kinetically limited dissolution of the light REs finallycatching up the dissolution of the heavy REs and 2) the formation ofREF₃ as a result of F⁻ being liberated into the IL.

Although the enrichment of the heavy REs (FIG. 9) occurs up to around 1hour, after this time the ratio of heavy REs to light REs decreasesquickly. The enrichment factor changes throughout the dissolutionprocess. This may be a result of the light REs (La—Nd) finally startingto have a greater uptake in the IL diluent. Increasing the scale of thedissolution experiment by a factor of 60 yielded a higher enrichmentfactor (0.37 heavy/light) than found on the smaller scale. This factorwas achieved after 15 minutes of the ore being in contact with theheated ionic liquid. FIG. 10 shows the fraction of heavy/light rareearths in the stripped samples as time progresses. Over the course of 4hours there was no substantial increase in the concentration of the rareearths in the samples; after 4 hours there was a 23% increase in theconcentration of the light rare earths and virtually no increase in theconcentration of the heavy rare earths.

This set of experiments was motivated by the froth flotation ore likelynot being homogeneous and that increasing the size of the startingsample would make consistency less of a concern. The results demonstratethat froth flotation bastnaesite is likely not homogeneous, also thatupscaling the process seems to affect the kinetics. The same weightratio of naturally occurring bastnaesite ore to IL was used for both thesmall- and large-scale experiments. It is possible for this ionic liquidsystem to be used with other rare-earth minerals and gangue minerals.

In conclusion, the dissolution process of bastnaesite-type solids in anionic liquid system has been demonstrated using the acidic amide ionicliquid (IL) N,N-dimethylacetamidium bis-(trifluoromethylsulfonyl)imide(DMAH⁺NTf₂ ⁻) in 1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide (BMIM⁺NTF₂ ⁻) secondary diluent. TheRECO₃F bastnaesite mineral was shown to consume two acidic protonsduring the dissolution process and to form both soluble RE³⁺ andinsoluble REF₃. The heavy REs (Eu—Ho, Y) dissolve faster than the lightREs (La—Nd) in this IL system, regardless of whether the materials aresingle-RECO₃F or natural bastnaesite, and the process allows heavy-REenrichment in the IL for a period of time. Although an IL system hasdensely concentrated ions and shows potential solvation capability andionic interactions with the RE³⁺ ions, the precipitation of REF₃ was notsuppressed. In addition, the liberation of CO₂ from the dissolutionprocess is necessary to push the reaction forward. The overalldissolution reaction proposed for this IL system is as follows:

3RECO₃F(s)+6DMAH⁺NTf ₂ ⁻→2RE(DMA)₃(NTf ₂)₃+REF₃(s)+3CO₂(g)+3H₂O

Dissolution of RECO₃F by Acidic Phosphorous Protic Ionic Liquids

After a rigorous study on the chemistry of dissolution and speciation ofthe interaction between RECO₃F and DMAH⁺NTf₂ ⁻ in BMIM⁺NTF₂ ⁻, acidicphosphorous protic ionic liquids were examined. See FIG. 1 forstructures of TBPH⁺ and TOPOH⁺ protonated phosphorus-containing species.The results of these studies are presented in Table 2 below. The resultsof these dissolution experiments were very similar; however, theTBPH⁺NTf₂ ⁻ IL, while it did dissolve the RECO₃F, degraded into ablackish brown liquid. Nevertheless, a promising result was that proticionic liquids containing TOPOH⁺, which is a protonated version ofCyanex®921, were competitive with the DMAH⁺NTf₂ ⁻/BMIM⁺NTF₂ ⁻ system,even at lower temperatures and contact times.

TABLE 2 Comparison of acidic amide and phosphorous-based protic ionicliquids Contact TSIL Solvent RECO₃F Temp (° C.) Time (h) [RE³⁺] (mol/kg)DMAH⁺NTf₂ ⁻ BMIM⁺NTf₂ ⁻ Nd₂O₃ 100 1 0.345 ± 0.012 DMAH⁺NTf₂ ⁻ BMIM⁺NTf₂⁻ NdCO₃F 120 1 0.12 ± 0.01 TBPH⁺NTf₂ ⁻ BMIM⁺NTf₂ ⁻ NdCO₃F 95 1.5 0.086 ±0.003 TBPH⁺NTf₂ ⁻ BMIM⁺NTf₂ ⁻ DyCO₃F 95 1.5 0.152 ± 0.009 TOPOH⁺NTf₂ ⁻OMIM⁺NTf₂ ⁻ NdCO₃F 95 1 0.0975 ± 0.013  Cyanex921- OMIM⁺NTf₂ ⁻ NdCO₃F 801 0.10 ± 0.02 HNTf₂ Cyanex921- BMIM⁺Cl⁻ NdCO₃F 80 1 0.11 ± 0.02 HCl

Systems further modified from the most analyzed system (DMAH⁺NTf₂⁻/BMIM⁺NTf₂ ⁻) appear to show similar behavior and RE dissolutionbehavior. In the example of the TOPOH⁺NTf₂ ⁻/OMIM₊NTf₂ ⁻ system, similarperformance was observed for dissolving the synthetic analogues ofbastnaesite. All of the phosphorous-based protic ionic liquids reportedin Table 2 likely possess good dissolution behavior by virtue of theirlow acid-dissociation constant (pKa).

While there have been shown and described what are at present consideredthe preferred embodiments of the invention, those skilled in the art maymake various changes and modifications which remain within the scope ofthe invention defined by the appended claims.

What is claimed is:
 1. A method for extracting a rare earth element froma rare earth-containing substance, the method comprising mixing the rareearth-containing substance with a protic ionic liquid having thefollowing formula:

wherein R¹ is selected from hydrogen and hydrocarbon groups containingat least 1 and up to 6 carbon atoms; R² and R³ are independentlyselected from hydrocarbon groups containing at least 1 and up to 12carbon atoms; and X⁻ is an anionic species; wherein said mixing resultsin a solution comprising a rare earth composition of the formula(RE)(amide)_(y)X_(z) at least partially dissolved in said protic ionicliquid, wherein RE is at least one rare earth element having an atomicnumber selected from 39, 57-71, and 90-103 and having a positiveoxidation state; y is 2-6; z is a number that charge balances the totalpositive charge of the at least one rare earth metals (RE); X isequivalent to X⁻ in the ionic liquid of Formula (1); and said amide isthe conjugate base of the cationic portion of the protic ionic liquid ofFormula (1) and has the following formula:


2. The method of claim 1, wherein X⁻ has a structure according to thechemical formula:

wherein m and n are independently 0 or an integer of 1 or above, and pis 0 or 1, provided that when p is 0, the group —N—SO₂—(CF₂)_(n)CF₃subtended by p is replaced with an oxide atom connected to the sulfuratom, and when p is 1, the shown perfluoroalkyl groups can optionallycrosslink to form a cyclic anion.
 3. The method of claim 1, wherein saidprotic ionic liquid is dissolved in a solvent, and said protic ionicliquid dissolved in the solvent is mixed with said rare earth-containingsubstance to result in at least partial dissolution of said rare earthcomposition comprising the formula (RE)(amide)_(y)X₃ into said solvent.4. The method of claim 3, wherein said solvent is a non-aqueous solvent.5. The method of claim 4, wherein said non-aqueous solvent is asecondary ionic liquid different from and non-reactive with the ionicliquid of Formula (1).
 6. The method of claim 5, wherein said secondaryionic liquid is a non-protic ionic liquid in which the ionic liquid ofFormula (1) is soluble.
 7. The method of claim 1, wherein the rareearth-containing substance comprises a carbonate or oxide of the rareearth element.
 8. The method of claim 1, wherein the rareearth-containing substance comprises at least one rare earth elementselected from atomic numbers of 39 and 57-71.
 9. The method of claim 8,wherein the rare earth-containing substance comprises bastnaesite. 10.The method of claim 1, wherein the rare earth-containing substancecomprises at least one of a first rare earth element selected fromlanthanum, cerium, praseodymium, and neodymium and at least one of asecond rare earth element selected from europium, terbium, dysprosium,holmium, and yttrium, and the protic ionic liquid dissolves at least oneof the second rare earth element to a greater degree than at least oneof the first rare earth element, thereby enriching the protic ionicliquid to a greater degree with at least one of the second rare earthelement than at least one of the first rare earth element.
 11. Themethod of claim 3, wherein the rare earth-containing substance comprisesat least one of a first rare earth element selected from lanthanum,cerium, praseodymium, and neodymium and at least one of a second rareearth element selected from europium, terbium, dysprosium, holmium, andyttrium, and the ionic liquid facilitates dissolution of at least one ofthe second rare earth element to a greater degree than at least one ofthe first rare earth element into the solvent, thereby enriching thesolvent to a greater degree with at least one of the second rare earthelement than at least one of the first rare earth element.
 12. Themethod of claim 3, wherein said mixing is conducted while the solvent inwhich the protic ionic liquid is dissolved is at a temperature of atleast 50° C.
 13. The method of claim 1, further comprising subjectingthe solution containing the rare earth composition of the formula(RE)(amide)_(y)X_(z) at least partially dissolved in said protic ionicliquid to an electroplating process in which the rare earth element iselectrodeposited from the solution.
 14. The method of claim 1, whereinthe protic ionic liquid is regenerated by reacting the amide conjugatebase of Formula (2) with a mineral acid.
 15. A method for extracting arare earth element from a rare earth-containing substance, the methodcomprising mixing the rare earth-containing substance with a proticionic liquid having the following formula:

wherein R⁴, R⁵, and R⁶ are independently selected from hydrocarbongroups (R) containing at least 1 and up to 12 carbon atoms and alkoxidegroups —OR; and X⁻ is an anionic species; wherein said mixing results ina solution comprising a rare earth composition of the formula(RE)(phos)_(y)X_(z) at least partially dissolved in said protic ionicliquid, wherein RE is at least one rare earth element having an atomicnumber selected from 39, 57-71, and 90-103 and having a positiveoxidation state; y is 2-6; z is a number that serves to charge balancethe total positive charge of the at least one rare earth metals (RE);and the phos group is the conjugate base of the cationic portion of theprotic ionic liquid of Formula (1) and has the following formula:


16. The method of claim 15, wherein X⁻ has a structure according to thechemical formula:

wherein m and n are independently 0 or an integer of 1 or above, and pis 0 or 1, provided that when p is 0, the group —N—SO₂—(CF₂)_(n)CF₃subtended by p is replaced with an oxide atom connected to the sulfuratom, and when p is 1, the shown perfluoroalkyl groups can optionallycrosslink to form a cyclic anion.
 17. The method of claim 14, whereinsaid protic ionic liquid is dissolved in a solvent, and said proticionic liquid dissolved in the solvent is mixed with said rareearth-containing substance to result in at least partial dissolution ofsaid rare earth composition comprising the formula (RE)(phos)_(y)X_(z)into said solvent.
 18. The method of claim 17, wherein said solvent is anon-aqueous solvent.
 19. The method of claim 18, wherein saidnon-aqueous solvent is a secondary ionic liquid different from andnon-reactive with the ionic liquid of Formula (4).
 20. The method ofclaim 19, wherein said secondary ionic liquid is a non-protic ionicliquid in which the ionic liquid of Formula (4) is soluble.
 21. Themethod of claim 15, wherein the rare earth-containing substancecomprises a carbonate or oxide of the rare earth element.
 22. The methodof claim 15, wherein the rare earth-containing substance comprises atleast one rare earth element selected from atomic numbers of 39 and57-71.
 23. The method of claim 22, wherein the rare earth-containingsubstance comprises bastnaesite.
 24. The method of claim 17, whereinsaid mixing is conducted while the solvent in which the protic ionicliquid is dissolved is at a temperature of at least 50° C.
 25. Themethod of claim 15, further comprising subjecting the solutioncontaining the rare earth composition of the formula (RE)(phos)_(y)X_(z)at least partially dissolved in said protic ionic liquid to anelectroplating process in which the rare earth element iselectrodeposited from the solution.
 26. The method of claim 15, whereinthe protic ionic liquid is regenerated by reacting the conjugate base ofFormula (5) with a mineral acid.
 27. A composition of the formula(RE)(amide)_(y)X_(z), wherein RE is at least one trivalent rare earthelement selected having an atomic number selected from 39, 57-71, and90-103; y is 2-6; z is a number that serves to charge balance the totalpositive charge of the at least one rare earth metals (RE); X is ananionic species; and said amide has the formula:

wherein R¹ is selected from hydrogen and hydrocarbon groups containingat least 1 and up to 12 carbon atoms; and R² and R³ are independentlyselected from hydrocarbon groups containing at least 1 and up to 12carbon atoms.
 28. The composition of claim 27, wherein X has theformula:

wherein m and n are independently 0 or an integer of 1 or above, and pis 0 or 1, provided that when p is 0, the group —N—SO₂—(CF₂)_(n)CF₃subtended by p is replaced with an oxide atom connected to the sulfuratom, and when p is 1, the shown perfluoroalkyl groups can optionallycrosslink to form a cyclic anion.
 29. A composition of the formula(RE)(phos)_(y)X₃, wherein RE is at least one trivalent rare earthelement having an atomic number selected from 39, 57-71, and 90-103; yis 2-6; X is an anionic species; z is a number that serves to chargebalance the total positive charge of the at least one rare earth metals(RE); and said phos group has the formula:

wherein R⁴, R⁵, and R⁶ are independently selected from hydrocarbongroups (R) containing at least 1 and up to 12 carbon atoms and alkoxidegroups —OR.
 30. The composition of claim 29, wherein X has the formula:

wherein m and n are independently 0 or an integer of 1 or above, and pis 0 or 1, provided that when p is 0, the group —N—SO₂—(CF₂)_(n)CF₃subtended by p is replaced with an oxide atom connected to the sulfuratom, and when p is 1, the shown perfluoroalkyl groups can optionallycrosslink to form a cyclic anion.